容器技术之Dockerfile（三）

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之所以要延迟多少秒做健康状态检查是因为，docker运行为容器以后，会立刻把该容器的状态标记为running状态，而对于有些初始化比较慢的容器，如果马上对它做健康状态检查，可能是不健康的状态，这样一来我们对了解容器是否健康就不是很准确了；如果配合某些工具，很可能存在检测到容器不健康就把该容器删除，然后重新创建，以此重复；这样就会导致我们的容器启动不起来；
前面我们聊到了dockerfile的 FROM、COPY 、ADD、LABEL、MAINTAINER、ENV、ARG、WORKDIR、VOLUME、EXPOSE、RUN、CMD、ENTRYPOINT指令的使用和说明，回顾请参考 https://www.cnblogs.com/qiuhom-1874/tag/Dockerfile/ ；今天我们来聊聊剩下的dockerfile指令的使用和说明；
1、USER：该指令用于指定运行image时的或运行dockerfile中任何RUN、CMD或ENTRYPOINT指令指定的程序时的用户名或UID；默认情况下，container的运行身份为root用户；语法格式 USER <UID>|<UserName>；需要注意的是，<UID>可以为任意数字，但实践中其必须为/etc/passwd中某用户的有效UID，否则，docker run命令将运行失败；
[root@node1 test]# cat Dockerfile 
FROM centos:7
LABEL maintainer="qiuhom <qiuhom@linux-1874.com>"
LABEL version="1.0"
LABEL description="this is test file \ that label-values can span multiple lines."
RUN useradd nginx
USER nginx
CMD ["sleep","3000"]
[root@node1 test]# 
　　提示：以上dockerfile表示在镜像运行成容器时，以nginx用户运行 sleep 3000
　　验证：编译成镜像，启动为容器，然后进入到容器里看看sleep 3000 是否是nginx用户在运行？
[root@node1 test]# docker build . -t test:v1
Sending build context to Docker daemon  1.051MB
Step 1/7 : FROM centos:7
 ---> b5b4d78bc90c
Step 2/7 : LABEL maintainer="qiuhom <qiuhom@linux-1874.com>"
 ---> Running in 0f503dae4448
Removing intermediate container 0f503dae4448
 ---> d31363b96f38
Step 3/7 : LABEL version="1.0"
 ---> Running in 8dad05999903
Removing intermediate container 8dad05999903
 ---> 2281f36d7c3c
Step 4/7 : LABEL description="this is test file \ that label-values can span multiple lines."
 ---> Running in d2be9ed44aee
Removing intermediate container d2be9ed44aee
 ---> 8de872e222fb
Step 5/7 : RUN useradd nginx
 ---> Running in 37bda6ba6b60
Removing intermediate container 37bda6ba6b60
 ---> dc681f95f5ca
Step 6/7 : USER nginx
 ---> Running in 97d2357826f9
Removing intermediate container 97d2357826f9
 ---> ed277ac0c482
Step 7/7 : CMD ["sleep","3000"]
 ---> Running in 0ea578fa10bc
Removing intermediate container 0ea578fa10bc
 ---> 461f6ceabc88
Successfully built 461f6ceabc88
Successfully tagged test:v1
[root@node1 test]# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
test                v1                  461f6ceabc88        3 seconds ago       204MB
centos              7                   b5b4d78bc90c        4 weeks ago         203MB
[root@node1 test]# docker run --name t1 --rm -d test:v1
37e46346d6ca0ab05b67f5350d4c2a7b6b86b8d34c8d1622d78ef70b7d3dff86
[root@node1 test]# docker ps 
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
37e46346d6ca        test:v1             "sleep 3000"        3 seconds ago       Up 2 seconds                            t1
[root@node1 test]# docker exec -it t1 /bin/bash
[nginx@37e46346d6ca /]$ ps aux
USER        PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
nginx         1  0.1  0.0   4364   352 ?        Ss   10:02   0:00 sleep 3000
nginx         6  0.4  0.0  11828  1808 pts/0    Ss   10:02   0:00 /bin/bash
nginx        23  0.0  0.0  51756  1708 pts/0    R+   10:02   0:00 ps aux
[nginx@37e46346d6ca /]$ exit
[root@node1 test]#
　　提示：可以看到基于上面的dockerfile构建的镜像运行为容器，里面默认跑的进程就是我们在dockerfile中指定用户运行的进程；使用USER指定用户运行容器里的进程，需要注意该用户要对运行进程所需资源的所有权限；否则容器运行不起来；
　　2、HEALTHCHECK：该指令用于定义如何对容器做健康状态检测；运行为容器后，容器里的进程不挂掉，当然容器也就不会挂掉，但是存在一种情况，容器没有挂掉，容器里的进程无法正常提供服务了，这个时候我们就需要通过一定的手段，第一时间知道容器里的进程是否健康（是否能够正常提供服务）；healthcheck指令就是用来定义如果去检测容器内部进程是否健康；语法格式HEALTHCHECK [OPTIONS] CMD command；其中CMD是固定格式，而后面的command是对容器里的进程做健康状态检查的命令；而options是用来指定对容器做健康状态检查的周期时间相关信息；--interval=DURATION (default: 30s)，该选项用于指定对容器做健康状态检查的频率，默认是30s一次；--timeout=DURATION (default: 30s)，该选项用于指定对容器内部的进程做健康状态检查的超时时长，默认是30秒；--start-period=DURATION (default: 0s)指定对容器中的进程做健康状态检查延迟时间，默认0表示不延迟；这里补充一点，之所以要延迟多少秒做健康状态检查是因为，docker运行为容器以后，会立刻把该容器的状态标记为running状态，而对于有些初始化比较慢的容器，如果马上对它做健康状态检查，可能是不健康的状态，这样一来我们对了解容器是否健康就不是很准确了；如果配合某些工具，很可能存在检测到容器不健康就把该容器删除，然后重新创建，以此重复；这样就会导致我们的容器启动不起来； --retries=N (default: 3)表示指定对容器做健康状态检查的重试次数，默认是3次；也就是说检查到容器不健康的前提或健康的前提，它都会检查3次,如果3次检查都是失败状态那么就标记该容器不健康；而对于我们指定的命令来讲，命令的返回值就决定了容器是否健康，通常命令返回值为0表示我们执行的命令正常退出，也就意味着容器是健康状态；命令返回值为1表示容器不健康；返回值为2我们通常都是保留不使用；HEALTHCHECK NONE就表示不对容器做健康状态检查；
[root@node1 test]# cat Dockerfile 
FROM centos:7
LABEL maintainer="qiuhom <qiuhom@linux-1874.com>"
LABEL version="1.0"
LABEL description="this is test file \ that label-values can span multiple lines."
RUN yum install -y httpd 
ADD ok.html /var/www/html/
CMD ["/usr/sbin/httpd","-DFOREGROUND"]
HEALTHCHECK --interval=5s --timeout=5s --start-period=5s --retries=2 \
        CMD curl -f http://localhost/ok.html || exit 1
[root@node1 test]# 
　　提示：以上HEALTHCHECK指令表示每5秒检查一次，超时时长为5秒，延迟5秒开始检查，重试2次；如果curl -f http://localhost/ok.html这条命令正常返回0，那么就表示容器健康，否则就返回1，表示容器不健康；
　　验证：把以上dockerfile构建成镜像启动为容器，我们把ok.html删除或移动到别的目录，看看容器是否标记为不健康？
[root@node1 test]# docker build . -t test:v1.1
Sending build context to Docker daemon  1.052MB
Step 1/8 : FROM centos:7
 ---> b5b4d78bc90c
Step 2/8 : LABEL maintainer="qiuhom <qiuhom@linux-1874.com>"
 ---> Using cache
 ---> d31363b96f38
Step 3/8 : LABEL version="1.0"
 ---> Using cache
 ---> 2281f36d7c3c
Step 4/8 : LABEL description="this is test file \ that label-values can span multiple lines."
 ---> Using cache
 ---> 8de872e222fb
Step 5/8 : RUN yum install -y httpd
 ---> Running in 9964718a2c3e
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
 * base: mirrors.bfsu.edu.cn
 * extras: mirrors.aliyun.com
 * updates: mirrors.aliyun.com
Resolving Dependencies
--> Running transaction check
---> Package httpd.x86_64 0:2.4.6-93.el7.centos will be installed
--> Processing Dependency: httpd-tools = 2.4.6-93.el7.centos for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: system-logos >= 7.92.1-1 for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: /etc/mime.types for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: libaprutil-1.so.0()(64bit) for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: libapr-1.so.0()(64bit) for package: httpd-2.4.6-93.el7.centos.x86_64
--> Running transaction check
---> Package apr.x86_64 0:1.4.8-5.el7 will be installed
---> Package apr-util.x86_64 0:1.5.2-6.el7 will be installed
---> Package centos-logos.noarch 0:70.0.6-3.el7.centos will be installed
---> Package httpd-tools.x86_64 0:2.4.6-93.el7.centos will be installed
---> Package mailcap.noarch 0:2.1.41-2.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
================================================================================
 Package             Arch          Version                    Repository   Size
================================================================================
Installing:
 httpd               x86_64        2.4.6-93.el7.centos        base        2.7 M
Installing for dependencies:
 apr                 x86_64        1.4.8-5.el7                base        103 k
 apr-util            x86_64        1.5.2-6.el7                base         92 k
 centos-logos        noarch        70.0.6-3.el7.centos        base         21 M
 httpd-tools         x86_64        2.4.6-93.el7.centos        base         92 k
 mailcap             noarch        2.1.41-2.el7               base         31 k
Transaction Summary
================================================================================
Install  1 Package (+5 Dependent packages)
Total download size: 24 M
Installed size: 32 M
Downloading packages:
warning: /var/cache/yum/x86_64/7/base/packages/apr-1.4.8-5.el7.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID f4a80eb5: NOKEY
Public key for apr-1.4.8-5.el7.x86_64.rpm is not installed
--------------------------------------------------------------------------------
Total                                              2.0 MB/s |  24 MB  00:12     
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
Importing GPG key 0xF4A80EB5:
 Userid     : "CentOS-7 Key (CentOS 7 Official Signing Key) <security@centos.org>"
 Fingerprint: 6341 ab27 53d7 8a78 a7c2 7bb1 24c6 a8a7 f4a8 0eb5
 Package    : centos-release-7-8.2003.0.el7.centos.x86_64 (@CentOS)
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : apr-1.4.8-5.el7.x86_64                                       1/6 
  Installing : apr-util-1.5.2-6.el7.x86_64                                  2/6 
  Installing : httpd-tools-2.4.6-93.el7.centos.x86_64                       3/6 
  Installing : centos-logos-70.0.6-3.el7.centos.noarch                      4/6 
  Installing : mailcap-2.1.41-2.el7.noarch                                  5/6 
  Installing : httpd-2.4.6-93.el7.centos.x86_64                             6/6 
  Verifying  : mailcap-2.1.41-2.el7.noarch                                  1/6 
  Verifying  : apr-util-1.5.2-6.el7.x86_64                                  2/6 
  Verifying  : httpd-2.4.6-93.el7.centos.x86_64                             3/6 
  Verifying  : apr-1.4.8-5.el7.x86_64                                       4/6 
  Verifying  : httpd-tools-2.4.6-93.el7.centos.x86_64                       5/6 
  Verifying  : centos-logos-70.0.6-3.el7.centos.noarch                      6/6 
Installed:
  httpd.x86_64 0:2.4.6-93.el7.centos                                            
Dependency Installed:
  apr.x86_64 0:1.4.8-5.el7                                                      
  apr-util.x86_64 0:1.5.2-6.el7                                                 
  centos-logos.noarch 0:70.0.6-3.el7.centos                                     
  httpd-tools.x86_64 0:2.4.6-93.el7.centos                                      
  mailcap.noarch 0:2.1.41-2.el7                                                 
Complete!
Removing intermediate container 9964718a2c3e
 ---> a931e93eea06
Step 6/8 : ADD ok.html /var/www/html/
 ---> 97e61f41911d
Step 7/8 : CMD ["/usr/sbin/httpd","-DFOREGROUND"]
 ---> Running in e91ccdef90c2
Removing intermediate container e91ccdef90c2
 ---> 7c8af9bb7eb3
Step 8/8 : HEALTHCHECK --interval=5s --timeout=5s --start-period=5s --retries=2         CMD curl -f http://localhost/ok.html || exit 1
 ---> Running in 80682ab087d3
Removing intermediate container 80682ab087d3
 ---> aa53cba15046
Successfully built aa53cba15046
Successfully tagged test:v1.1
[root@node1 test]# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
test                v1.1                aa53cba15046        8 seconds ago       312MB
test                v1                  461f6ceabc88        57 minutes ago      204MB
centos              7                   b5b4d78bc90c        4 weeks ago         203MB
[root@node1 test]# docker run --name t1 --rm -d test:v1.1
332590e683fcb29f60a28703548fce7aa83df715cbb840e1283472834867d6a1
[root@node1 test]# docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                            PORTS               NAMES
332590e683fc        test:v1.1           "/usr/sbin/httpd -DF…"   3 seconds ago       Up 2 seconds (health: starting)                       t1
[root@node1 test]# docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                   PORTS               NAMES
332590e683fc        test:v1.1           "/usr/sbin/httpd -DF…"   7 seconds ago       Up 6 seconds (healthy)                       t1
[root@node1 test]# 
　　提示：可以看到基于我们写的dockerfile构建的镜像已经成功运行为容器，并且标记为healthy；接下来我们进入容器把ok.html干掉，然后在看看容器是否标记为不健康状态？
　　提示：从上面的信息可以看到我们把ok.html移除后，容器状态就变成不健康状态了；我们再把ok.html还原到原有位置，看看容器是否会从不健康转换为健康呢？
　　提示：可以看到把ok.html还原到/var/www/html/目录后，容器从不健康状态变为了健康状态；
　　3、SHELL：该指令用于指定默认shell，该指令开始到下一个SHELL中间的命令都是SHELL指定的shell 运行，所以SHELL指令在dockerfile中可出现多次，后面的SHELL指令指定的shell会覆盖前面所有SHELL指令指定的shell；默认在Linux上是["/bin/sh","-c"]在Windows上述["cmd","/s","/c"];SHELL指令必须是以json数组的格式定义；语法SHELL ["executable", "parameters"]；
　　4、STOPSIGNAL：该指令用于定义停止容器的信号；默认停止容器是15号信号 SIGTERM；语法STOPSIGNAL signal
　　5、ONBUILD：该指令用于在Dockerfile中定义一个触发器；Dockerfile用于build映像文件，此映像文件亦可作为base image被另一个Dockerfile用作FROM指令的参数，并以之构建新的映像文件；在后面的这个Dockerfile中的FROM指令在build过程中被执行时，将会“触发”创建其base image的Dockerfile文件中的ONBUILD指令定义的触发器；用法格式ONBUILD <INSTRUCTION>；尽管任何指令都可注册成为触发器指令，但ONBUILD不能自我嵌套，且不会触发FROM和MAINTAINER指令；使用包含ONBUILD指令的Dockerfile构建的镜像应该使用特殊的标签，例如ruby:2.0-onbuild；在ONBUILD指令中使用ADD或COPY指令应该格外小心，因为新构建过程的上下文在缺少指定的源文件时会失败；
[root@node1 test]# cat Dockerfile
FROM centos:7
LABEL maintainer="qiuhom <qiuhom@linux-1874.com>"
ONBUILD RUN yum install -y httpd
　　提示：以上dockerfile表示在本次构建镜像中不运行yum install -y httpd这条命令，而是在后面的dockerfile中以本dockerfile制作的进行作为基础继续时，yum install -y httpd这条命令就会被触发执行；简单讲onbuild就是指定dockerfile指令延迟执行；这里一定要记住一点onbuild指令后面一定是跟的是dockerfile指令；
　　验证：将上面的dockerfile编译镜像，看看yum install -y httpd 是否执行了？
[root@node1 test]# docker build . -t test:v1.5
Sending build context to Docker daemon  1.052MB
Step 1/3 : FROM centos:7
 ---> b5b4d78bc90c
Step 2/3 : LABEL maintainer="qiuhom <qiuhom@linux-1874.com>"
 ---> Using cache
 ---> d31363b96f38
Step 3/3 : ONBUILD RUN yum install -y httpd
 ---> Running in d3601fa1c3b7
Removing intermediate container d3601fa1c3b7
 ---> 370e3a843c3c
Successfully built 370e3a843c3c
Successfully tagged test:v1.5
[root@node1 test]# 
　　提示：可以看到yum install -y httpd 这条命令并没有执行；
　　验证：将我们上面制作好的镜像作为基础镜像，再来制作其他镜像，看看yum install -y httpd 被执行？
[root@node1 aaa]# pwd
/root/test/aaa
[root@node1 aaa]# ls
Dockerfile
[root@node1 aaa]# cat Dockerfile 
FROM test:v1.5
LABEL maintainer="qiuhom <admin@admin.com>"
[root@node1 aaa]# docker build . -t myweb:v1
Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM test:v1.5
# Executing 1 build trigger
 ---> Running in cf93e9f03e89
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
 * base: mirrors.huaweicloud.com
 * extras: mirrors.aliyun.com
 * updates: mirrors.aliyun.com
Resolving Dependencies
--> Running transaction check
---> Package httpd.x86_64 0:2.4.6-93.el7.centos will be installed
--> Processing Dependency: httpd-tools = 2.4.6-93.el7.centos for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: system-logos >= 7.92.1-1 for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: /etc/mime.types for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: libaprutil-1.so.0()(64bit) for package: httpd-2.4.6-93.el7.centos.x86_64
--> Processing Dependency: libapr-1.so.0()(64bit) for package: httpd-2.4.6-93.el7.centos.x86_64
--> Running transaction check
---> Package apr.x86_64 0:1.4.8-5.el7 will be installed
---> Package apr-util.x86_64 0:1.5.2-6.el7 will be installed
---> Package centos-logos.noarch 0:70.0.6-3.el7.centos will be installed
---> Package httpd-tools.x86_64 0:2.4.6-93.el7.centos will be installed
---> Package mailcap.noarch 0:2.1.41-2.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
================================================================================
 Package             Arch          Version                    Repository   Size
================================================================================
Installing:
 httpd               x86_64        2.4.6-93.el7.centos        base        2.7 M
Installing for dependencies:
 apr                 x86_64        1.4.8-5.el7                base        103 k
 apr-util            x86_64        1.5.2-6.el7                base         92 k
 centos-logos        noarch        70.0.6-3.el7.centos        base         21 M
 httpd-tools         x86_64        2.4.6-93.el7.centos        base         92 k
 mailcap             noarch        2.1.41-2.el7               base         31 k
Transaction Summary
================================================================================
Install  1 Package (+5 Dependent packages)
Total download size: 24 M
Installed size: 32 M
Downloading packages:
warning: /var/cache/yum/x86_64/7/base/packages/apr-1.4.8-5.el7.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID f4a80eb5: NOKEY
Public key for apr-1.4.8-5.el7.x86_64.rpm is not installed
--------------------------------------------------------------------------------
Total                                              7.2 MB/s |  24 MB  00:03     
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
Importing GPG key 0xF4A80EB5:
 Userid     : "CentOS-7 Key (CentOS 7 Official Signing Key) <security@centos.org>"
 Fingerprint: 6341 ab27 53d7 8a78 a7c2 7bb1 24c6 a8a7 f4a8 0eb5
 Package    : centos-release-7-8.2003.0.el7.centos.x86_64 (@CentOS)
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : apr-1.4.8-5.el7.x86_64                                       1/6 
  Installing : apr-util-1.5.2-6.el7.x86_64                                  2/6 
  Installing : httpd-tools-2.4.6-93.el7.centos.x86_64                       3/6 
  Installing : centos-logos-70.0.6-3.el7.centos.noarch                      4/6 
  Installing : mailcap-2.1.41-2.el7.noarch                                  5/6 
  Installing : httpd-2.4.6-93.el7.centos.x86_64                             6/6 
  Verifying  : mailcap-2.1.41-2.el7.noarch                                  1/6 
  Verifying  : apr-util-1.5.2-6.el7.x86_64                                  2/6 
  Verifying  : httpd-2.4.6-93.el7.centos.x86_64                             3/6 
  Verifying  : apr-1.4.8-5.el7.x86_64                                       4/6 
  Verifying  : httpd-tools-2.4.6-93.el7.centos.x86_64                       5/6 
  Verifying  : centos-logos-70.0.6-3.el7.centos.noarch                      6/6 
Installed:
  httpd.x86_64 0:2.4.6-93.el7.centos                                            
Dependency Installed:
  apr.x86_64 0:1.4.8-5.el7                                                      
  apr-util.x86_64 0:1.5.2-6.el7                                                 
  centos-logos.noarch 0:70.0.6-3.el7.centos                                     
  httpd-tools.x86_64 0:2.4.6-93.el7.centos                                      
  mailcap.noarch 0:2.1.41-2.el7                                                 
Complete!
Removing intermediate container cf93e9f03e89
 ---> a89914bda4b5
Step 2/2 : LABEL maintainer="qiuhom <admin@admin.com>"
 ---> Running in e175e0542b5e
Removing intermediate container e175e0542b5e
 ---> 4f406abeaab7
Successfully built 4f406abeaab7
Successfully tagged myweb:v1
[root@node1 aaa]#
　　提示：可以看到在我们的dockerfile中并没有写 RUN  yum install -y httpd  ，但build时却执行了 yum install -y httpd ；这是因为onbuild指令被触发了；我们可以理解为如果我们制作的镜像有onbuild指令指定的命令，那么该镜像被其他dockerfile 作为基础镜像时（或者被其他docker FROM指令引用时）onbuild指定就会被激活，被执行；
        作者：Linux-1874
        出处：https://www.cnblogs.com/qiuhom-1874/
        本文版权归作者和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利.