docker bash 에서 env 변수를 에코하는 방법
docker exec <container-name> env
도커 ARG
$ docker build --build-arg some_variable_name=a_value
dockerfile env variabes
FROM busybox
ENV foo /bar
WORKDIR ${foo} # WORKDIR /bar
ADD . $foo # ADD . /bar
COPY \$foo /quux # COPY $foo /quux