HOW TO use env variable in dockerfile code example

Example 1: docker how to echo env variable in bash

docker exec <container-name> env

Example 2: dockerfile env

ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc

Example 3: dockerfile env variabes

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