dockerfile中ONBUILD指令的功能简介说明

下文笔者讲述dockerfile中ONBUILD命令的功能简介说明，如下所示

ONBUILD指令简介

ONBUILD是一个特殊的指令
 它后面跟的是其它指令
  如 RUN, COPY 等
     这些指令在当前镜像构建时并不会被执行
  以当前镜像为基础镜像
  去构建下一级镜像的时候
  才会执行ONBUILD后面的指令

ONBUILD指令的使用

例:
创建Dockerfile文件
Dockerfile_test 内容如下

FROM centos  # 初始镜像
RUN yum install -y curl  # 执行安装curl命令
ENTRYPOINT ["curl", "-s", "http://ip.cn"] # 容器启动后执行的命令
ONBUILD RUN echo "father---image---onbuild====886"

如:
 假设有哪一个镜像继承了该镜像(父镜像)
  当子镜像build的时候,就会触发父镜像的ONBUILD指令
使用Dockerfile_onbuild文件生成onbuild/father镜像，如下：

[root@192 mydockerfile]# docker build -f /home/mydockerfile/Dockerfile_test -t onbuild/father:1.0 .
Sending build context to Docker daemon   5.12kB
Step 1/4 : FROM centos
 ---> 8129tde21s0d
Step 2/4 : RUN yum install -y curl
 ---> Running in 11cd05afe92c
CentOS Linux 8 - AppStream                      2.5 MB/s | 6.3 MB     00:02    
CentOS Linux 8 - BaseOS                         1.0 MB/s | 2.3 MB     00:02    
CentOS Linux 8 - Extras                         2.3 kB/s | 9.2 kB     00:03    
Package curl-7.61.1-14.el8.x86_64 is already installed.
Dependencies resolved.
================================================================================
 Package               Architecture Version                  Repository    Size
================================================================================
Upgrading:
 curl                  x86_64       7.61.1-14.el8_3.1        baseos       353 k
 libcurl-minimal       x86_64       7.61.1-14.el8_3.1        baseos       285 k

Transaction Summary
================================================================================
Upgrade  2 Packages

Total download size: 638 k
Downloading Packages:
(1/2): curl-7.61.1-14.el8_3.1.x86_64.rpm        546 kB/s | 353 kB     00:00    
(2/2): libcurl-minimal-7.61.1-14.el8_3.1.x86_64 406 kB/s | 285 kB     00:00    
--------------------------------------------------------------------------------
Total                                           547 kB/s | 638 kB     00:01     
CentOS Linux 8 - BaseOS                         1.6 MB/s | 1.6 kB     00:00    
warning: /var/cache/dnf/baseos-f6a80ba95cf937f2/packages/curl-7.61.1-14.el8_3.1.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID 8483c65d: NOKEY
Importing GPG key 0x8483C65D:
 Userid     : "CentOS (CentOS Official Signing Key) "
 Fingerprint: 99DB 70FA E1D7 CE22 7FB6 4882 05B5 55B3 8483 C65D
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
Key imported successfully
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        :                                                        1/1 
  Upgrading        : libcurl-minimal-7.61.1-14.el8_3.1.x86_64               1/4 
  Upgrading        : curl-7.61.1-14.el8_3.1.x86_64                          2/4 
  Cleanup          : curl-7.61.1-14.el8.x86_64                              3/4 
  Cleanup          : libcurl-minimal-7.61.1-14.el8.x86_64                   4/4 
  Running scriptlet: libcurl-minimal-7.61.1-14.el8.x86_64                   4/4 
  Verifying        : curl-7.61.1-14.el8_3.1.x86_64                          1/4 
  Verifying        : curl-7.61.1-14.el8.x86_64                              2/4 
  Verifying        : libcurl-minimal-7.61.1-14.el8_3.1.x86_64               3/4 
  Verifying        : libcurl-minimal-7.61.1-14.el8.x86_64                   4/4 

Upgraded:
  curl-7.61.1-14.el8_3.1.x86_64     libcurl-minimal-7.61.1-14.el8_3.1.x86_64    

Complete!
Removing intermediate container 11cd05afe92c
 ---> c88555f994f9
Step 3/4 : ENTRYPOINT ["curl", "-s", "http://ip.cn"]
 ---> Running in 9d9061a8d47e
Removing intermediate container 9d9061a8d47e
 ---> 96a5f70b56bf
Step 4/4 : ONBUILD RUN echo "father---image---onbuild====886"
 ---> Running in 68ca17cc9a0b
Removing intermediate container 68ca17cc9a0b
 ---> 05fdd64f3e60
Successfully built 05fdd64f3e60
Successfully tagged onbuild/father:1.0

# 查看本地Docker镜像
[root@192 mydockerfile]# docker images
REPOSITORY          TAG       IMAGE ID       CREATED          SIZE
onbuild/father      1.0       05fdd64f3e60   42 seconds ago   243MB
centos              latest    300e315adb2f   3 months ago     209MB
（2）开始演示

我们再创建一个Dockerfile文件：Dockerfile_onbuild_son。

[root@192 mydockerfile]# pwd
/home/mydockerfile
[root@192 mydockerfile]# touch Dockerfile_onbuild_son
编辑Dockerfile_onbuild_son文件内容如下：

# 这里要继承上面生成的onbuild/father镜像
# 这样onbuild/father镜像就成了父镜像
# 通过该Dockerfile文件生成的镜像就是子镜像。
FROM onbuild/father:1.0  # 注意这里一定要加TAG版本，不然无法找到该镜像。 
RUN yum install -y curl  # 执行安装curl命令

CMD /bin/bash
使用Dockerfile_onbuild_son文件生成onbuild/son镜像，如下：

[root@192 mydockerfile]# docker build -f /home/mydockerfile/Dockerfile_onbuild_son -t onbuild/son:1.0 .
Sending build context to Docker daemon  6.144kB
Step 1/3 : FROM onbuild/father:1.0
# Executing 1 build trigger       # 看这三行，就是ONBUILD指令的效果
 ---> Running in 0a165ca6352c     # Executing 1 build trigger 执行1个构建触发器
father---image---onbuild====886   # 数据了父镜像ONBUILD指令执行的内容   
Removing intermediate container 0a165ca6352c
 ---> f9313959d1ae
Step 2/3 : RUN yum install -y curl
 ---> Running in 0485b0d9d031
Last metadata expiration check: 0:09:32 ago on Sat Mar 20 14:46:45 2021.
Package curl-7.61.1-14.el8_3.1.x86_64 is already installed.
Dependencies resolved.
Nothing to do.
Complete!
Removing intermediate container 0485b0d9d031
 ---> 0718c9a2d99a
Step 3/3 : CMD /bin/bash
 ---> Running in e102da5c1b5e
Removing intermediate container e102da5c1b5e
 ---> 46ed53584363
Successfully built 46ed53584363
Successfully tagged onbuild/son:1.0

以上就是ONBUILD指令的使用方式和效果。