关于dockerfile语法问题

Question

看一个 dockerfile 文件，前面通过 ARG 和 ENV 设置了变量

ARG INSTALL_WORKSPACE_SSH=false
ENV INSTALL_WORKSPACE_SSH ${INSTALL_WORKSPACE_SSH}

后面又通过读取环境变量的方式判断 INSTALL_WORKSPACE_SSH 的值

if [ ${INSTALL_WORKSPACE_SSH} = true ]

这样不是有问题吗？前面已经定义了值，那么后面再读取环境有什么意义？

#####################################
# ssh:
#####################################
ARG INSTALL_WORKSPACE_SSH=false
ENV INSTALL_WORKSPACE_SSH ${INSTALL_WORKSPACE_SSH}

ADD insecure_id_rsa /tmp/id_rsa
ADD insecure_id_rsa.pub /tmp/id_rsa.pub

RUN if [ ${INSTALL_WORKSPACE_SSH} = true ]; then \
    rm -f /etc/service/sshd/down && \
    cat /tmp/id_rsa.pub >> /root/.ssh/authorized_keys \
        && cat /tmp/id_rsa.pub >> /root/.ssh/id_rsa.pub \
        && cat /tmp/id_rsa >> /root/.ssh/id_rsa \
        && rm -f /tmp/id_rsa* \
        && chmod 644 /root/.ssh/authorized_keys /root/.ssh/id_rsa.pub \
    && chmod 400 /root/.ssh/id_rsa \
;fi

Answer 1

arg指令定义的变量只在编译期间有效，同时可以在build时通过外部参数 —build-arg 名称=值覆盖。所以还需要重新判断是否是期望的值