ONBUILD是 当目前的镜像在基于父镜像的基础上创建的时候，执行了父镜像的onbuild语句，
    FROM 父镜像 而不是 创建父镜像的dockerfile

    1. //编写父镜像
    2. [root@test mydocker]# cat fatherfile
    3. FROM centos
    4. RUN yum install -y curl
    5. ENTRYPOINT ["curl","-s","http://ip.cn"]
    6. ONBUILD RUN echo  "father is good"
    8. //创建father名称的镜像
    9. [root@test mydocker]# docker build -f /mydocker/fatherfile -t father .
    10. Sending build context to Docker daemon  6.144kB
    11. Step 1/4 : FROM centos
    12.  ---> 300e315adb2f
    13. Step 2/4 : RUN yum install -y curl
    14.  ---> Using cache
    15.  ---> 5f37af576672
    16. Step 3/4 : ENTRYPOINT ["curl","-s","http://ip.cn"]
    17.  ---> Using cache
    18.  ---> adf1e62f8da6
    19. Step 4/4 : ONBUILD RUN echo  "father is good"
    20.  ---> Using cache
    21.  ---> fbb09770b5ed
    22. Successfully built fbb09770b5ed
    23. Successfully tagged father:latest
    26. //编写子dockerfile文件继承父镜像
    27. [root@test mydocker]# cat sonfile 
    29. //子镜像的dockerfile继承的是创建出来的父镜像，而不是创建父镜像用的dockerfile文件
    30. FROM father      
    31. RUN yum install -y curl
    32. ENTRYPOINT ["curl","-s","http://ip.cn"]
    33. ONBUILD RUN echo  "father is good"
    35. //创建子镜像
    36. [root@test mydocker]# docker build -f /mydocker/sonfile -t son . 
    37. Sending build context to Docker daemon  7.168kB
    39. //继承父镜像
    40. Step 1/4 : FROM father    
    41. # Executing 1 build trigger
    42.  ---> Running in 7ca9d7f848c9
    44. //执行了创建父镜像的onbuild语句
    45. father is good                
    46. Removing intermediate container 7ca9d7f848c9
    47.  ---> a480887d7a79
    48. Step 2/4 : RUN yum install -y curl
    49.  ---> Running in b57ba10115fc
    50. Last metadata expiration check: 8:12:35 ago on Tue Jan 26 14:51:14 2021.
    51. Package curl-7.61.1-14.el8_3.1.x86_64 is already installed.
    52. Dependencies resolved.
    53. Nothing to do.
    54. Complete!
    55. Removing intermediate container b57ba10115fc
    56.  ---> eaa5d942d3a2
    57. Step 3/4 : ENTRYPOINT ["curl","-s","http://ip.cn"]
    58.  ---> Running in c5c07c614ee2
    59. Removing intermediate container c5c07c614ee2
    60.  ---> 89ef8472381d
    61. Step 4/4 : ONBUILD RUN echo  "father is good"
    62.  ---> Running in 02fa4990dd3f
    63. Removing intermediate container 02fa4990dd3f
    64.  ---> f0d5a065fb1e
    65. Successfully built f0d5a065fb1e
    66. Successfully tagged son:latest
    67. [root@test mydocker]#