how to pass host user to Dockerfile when using docker-compose - command-line

Question

I'm trying to pass a host variable to a Dockerfile when running docker-compose build
I would like to run
RUN usermod -u $USERID www-data
in an apache-php7 Dockerfile. $USERID being the ID of the current host user.
I would have thought that the following might work:
commandline
EXPORT USERID=$(id -u); docker-compose build
docker-compose.yml
...
environment:
- USERID=$USERID
Dockerfile
ENV USERID
RUN usermod -u $USERID www-data
But no luck yet.

Answer 1

For Docker in general it is generally not possible to use host environment variables during the build phase; this is because it is desirable that if you run docker build and I run docker build using the same Dockerfile (or Docker Hub runs `docker build with the same Dockerfile), we end up with the same image, regardless of our local environment.
While passing in variables at runtime is easy with the docker command line (using -e <var>=<value>), it's a little trickier with docker-compose, because that tool is designed to create self-contained environments.
A simple solution would be to drop the host uid into an environment file before starting the container. That is, assuming you have:
version: "2"
services:
shell:
image: alpine
env_file: docker-compose.env
command: >
env
You can then:
echo HOST_UID=$UID > docker-compose.env; docker-compose up
And the HOST_UID environment variable will be available to your
container:
Recreating vartest_shell_1
Attaching to vartest_shell_1
shell_1 | HOSTNAME=17423d169a25
shell_1 | HOST_UID=1000
shell_1 | HOME=/root
vartest_shell_1 exited with code 0
You would then to have something like an ENTRYPOINT script that
would set up the container environment (creating users, modifying file
ownership) to operate correctly with the given UID.