前面介绍了dockerfile的常用命令，以及如何构建自己的centos rootfs, 这里用前面学习的内容，来创建 自己的base image .
该dockerfile包含如下的内容:
a. 安装相应的RPM包，并用useradd创建用户, docker build的时候可以通过--build-arg=USERNAME=[USER_NAME] 来指定用户名称，如果在docker build的时候没有指定，USERNAME的默认值为 PandaEye；
b. 设置EntryPoint指令值为/bin/bash；CMD指令的值welcome.sh作为ENTRYPOING指令的参数；
c. 在该base image中还需要指定ONBUILD 指令，以便基于该base image的使用者知道该base image的一些信息。
下面演示创建该base image的过程：
1.
创建welcome.sh 文件：
[root@localhost ~]# cat welcome.sh
#!/bin/bash
echo -e "\033[1;32;40mUSAGE:\nYou can override the environment 'USERNAME' with your favorite name.\nAlso,you can override the instruction value of 'CMD' with any available commands in the system by format '-c COMMAND'.\033[0m"
export USERNAME
if [[ -z $USERNAME ]];then
export USERNAME="Private_PandaEye"
fi
echo -e "\nWelcome to the Docker World,$USERNAME.\n"
[root@localhost ~]#
2.
创建dockerfile文件，文件内容如下：
[root@localhost ~]# cat dockerfile
#This is the 1st dockerfile for study dockerfile syntax.
FROM scratch
MAINTAINER PandaEye
#Add the archive tar.gz file to the / of the docker images. The image generated after this instuctions.
ADD ./my_rootfs.tar.gz /
#Install the package so that the "useradd" commands is available.
#Here, while we use the exec format for RUN instructions,environment variable can not be parsed,because no shell will be used.
#So we need use the sh or bash in exec format to make the environment variable can be parsed.
#"/bin/bash -c yum install $PKGNAME -y" inlcude 3 parts： command "/bin/bash", option "-c" , option value "yum install $PKGNAME -y"
#This 3 part can not be change. otherwise, it report error.
ARG PKGNAME="shadow-utils"
RUN ["/bin/bash","-c","yum install $PKGNAME -y"]
#Create the user,the default username is 'PandaEye', it can be override by:"docker build --build-arg=USERNAME=WANTED_NAME".
ARG USERNAME=PandaEye
RUN useradd -c "$USERNAME's ID" -b /bin/bash -m -d /home/$USERNAME $USERNAME
WORKDIR /home/$USERNAME
#In the file : welcome.sh , it try to use the ENV parameter: USERNAME,if not defined, it will define in the welcome.sh
COPY ./welcome.sh ./
#Below environment USERNAME value is ARG's USERNAME value. If no this line, the docker images has no environment USERNAME.
#The USERNAME environment parameter can be override by: "docker run --env USERNAME=value".
ENV USERNAME $USERNAME
#The CMD can be override while using: docker run IMAGNAME DEFINED_CMD , the DEFINED_CMD must have the similar format: -c CMD_VALUE
#Both CMD and ENTRYPOINT using the exec-format required for make CMD as parameter of ENTRYPOINT.
CMD ["-c","./welcome.sh"]
ENTRYPOINT ["/bin/bash"]
#ONBUILD instruction used to set the info for new image which based on this image.
ONBUILD ARG BASEIMG_RD="The base image build by auth: PandaEye. This is not official base image. "
ONBUILD RUN ["/bin/bash","-c","echo $BASEIMG_RD"]
[root@localhost ~]#
3.
用docker build -t my_baseimage 来build image. 命令如下：
[root@localhost ~]# docker build -t my_baseimage --build-arg=USERNAME="ZHANGFEI" .
Sending build context to Docker daemon 725.3 MB
Step 1/14 : FROM scratch
--->
Step 2/14 : MAINTAINER PandaEye
---> Running in fa6cd58beed7
......
Removing intermediate container 9386d4f58307
Step 14/14 : ONBUILD run /bin/bash -c echo $BASEIMG_RD
---> Running in c367b9387a11
---> e64162ad76f3
Removing intermediate container c367b9387a11
Successfully built e64162ad76f3
[root@localhost ~]#
4.
用docker run来测试容器：
a), ARG参数USERNAME的默认值已经在docker build时候通过--build-arg=USERNAME=ZHANGFEI来替换，结果如下：
[root@localhost ~]# docker run my_baseimage
USAGE:
You can override the environment 'USERNAME' with your favorite name.
Also,you can override the instruction value of 'CMD' with any available commands in the system by format '-c COMMAND'.
Welcome to the Docker World,ZHANGFEI.
b), 在docker run的时候，通过 -e USERNAME参数来替换dockerfile中指定的ENV参数USERNAME的值，注意ARG的USERNAME无法作用于docker run时候引用的USERNAME，但是ENV的USERNAME是可以的：
[root@localhost ~]# docker run -e USERNAME=GuanYu my_baseimage
USAGE:
You can override the environment 'USERNAME' with your favorite name.
Also,you can override the instruction value of 'CMD' with any available commands in the system by format '-c COMMAND'.
Welcome to the Docker World,GuanYu.
c), 通过docker run时候替换CMD指令的值，从而进入容器的内部：
[root@localhost ~]# docker run -it my_baseimage -c /bin/bash
bash-4.2# ls
welcome.sh
bash-4.2# echo $USERNAME
ZHANGFEI
bash-4.2# exit
exit
[root@localhost ~]# docker run -it -e USERNAME=LIUBEI my_baseimage -c /bin/bash
bash-4.2# echo $USERNAME
LIUBEI
bash-4.2#
这里介绍该dockerfile中的几个值得注意的地方：
A. 对于RUN,CMD,ENTRYPOIN等指令参数书写的时候，有exec 格式和shell格式；其中引用环境变量的时候，shell格式可以直接引用；而exec格式是不可以的，因为exec格式不会调用 shell, 但是对环境变量的解析是shell来完成的，所以要在exec格式中引用环境变量，要使用 ["/bin/bash","-c","PARAMETER"] 的格式；
这里重点在于对于PARAMETER的理解，“-c” 是 /bin/bash的参数，PARAMETER就是参数 “-c” 的值，所以是一个整体，不可以拆分成多个部分，比如：
RUN ["/bin/bash","-c","yum install $PKGNAME -y"] 是正确的写法，而如果进一步拆分，比如RUN ["/bin/bash","-c","yum", "install $PKGNAME"," -y"] 就是错误的，因为 只有yum 作为了-c参数的值，其他是/bin/bash无法识别的参数,所以会报错；
B. ARG参数 USERNAME和ENV参数USERNAME虽然同名，但是ARG参数USENAME在docker run时候是不可见的；只有ENV参数USERNAME才是docker run时候可以访问的；
C. 当用CMD指令的参数值来作为 ENTRYPOING指令的默认参数的时候，那么需要注意以下两点：
1). ENTRYPOINT的值必须是 可以正确执行的，比如 /bin/bash, 不可以是 /bin/bash -c
2). ENTRYPOINT 和 CMD 都必须使用 exec格式；
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