ONBUILD说明：
    当构建一个被继承的DockerFile时运行命令，父镜像在被子镜像继承后父镜像的onbuild被触发，只有build时

    1. FROM centos:7
    2. RUN yum install -y curl
    3. CMD curl -s https://www.ip.cn/
    4. ONBUILD RUN echo "father images onbuild!!!!!!!!!!!!!!!!!!!"
    1. FROM ip_father
    1. [linguo@VM-4-6-centos dockerFile]$ docker build -f dockerfile_ipson -t ip_son .
    2. Sending build context to Docker daemon  11.26kB
    3. Step 1/2 : FROM ip_father
    4. # Executing 1 build trigger
    5.  ---> Running in 83509019d77b
    6. father images onbuild!!!!!!!!!!!!!!!!!!!
    7. Removing intermediate container 83509019d77b
    8.  ---> a9f2d80c3663
    9. Step 2/2 : CMD ["-i"]
    10.  ---> Running in fa53adf0f9f7
    11. Removing intermediate container fa53adf0f9f7
    12.  ---> 0fdfdd432943
    13. Successfully built 0fdfdd432943
    14. Successfully tagged ip_son:latest

    看到打印了父镜像的这句话“father images onbuild!!!!!!!!!!!!!!!!!!!”