dokcer how to get commands in an official image code example

Example 1: docker ARG

$ docker build --build-arg some_variable_name=a_value

Example 2: dockerfile env variabes

FROM busybox
ENV foo /bar
WORKDIR ${foo}   # WORKDIR /bar
ADD . $foo       # ADD . /bar
COPY \$foo /quux # COPY $foo /quux