Q:

Conditional logic in Dockerfile, using --build-arg

Say I have this:
ARG my_user="root"  # my_user => default is "root"
USER $my_user
ENV USER=$my_user

All good so far, but now we get here:
ENV HOME="/root"

is there a way to do something like this:
ENV HOME $my_user === "root"? "/root" : "/home/$my_user"

Obviously, that's the wrong syntax.
The only solution I can think of is to just use two --build-args, something like this:
docker build -t zoom \
    --build-arg my_user="foo"  \
    --build-arg my_home="/home/foo"  \
     .

A:

Unfortunately you can't do this directly
https://forums.docker.com/t/how-do-i-send-runs-output-to-env-in-dockerfile/16106/3
So you have two alternatives
Use a shell script at start
You can use a shell script at the start
CMD /start.sh

And in your start.sh you can have that logic
if [ $X == "Y" ]; then
   export X=Y
else
   export X=Z
fi

Create a profile environment variable
FROM alpine

RUN echo "export NAME=TARUN" > /etc/profile.d/myenv.sh
SHELL ["/bin/sh", "-lc"]
CMD env

And then you when you run it
$ docker run test
HOSTNAME=d98d44fa1dc9
SHLVL=1
HOME=/root
PAGER=less
PS1=\h:\w\$
NAME=TARUN
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/
CHARSET=UTF-8

Note: The SHELL ["/bin/sh", "-lc"] is quite important here, else the profile will not be loaded
Note2: Instead of RUN echo "export NAME=TARUN" > /etc/profile.d/myenv.sh you can also do a COPY myevn.sh /etc/profile.d/myenv.sh and have the file be present in your build context

