Docker-compose, conditional statements? (e.g. add volume only if condition)

Question

I want to add a volume to my service, but only if the final user gave a folder for it. Otherwise, no volume should be mounted, for the already-prepared image has valid data in a

Answer 1

Poor man's solution:

    volumes:
      ${VARIABLE:-/dev/null}:/app/folder

Or:

    volumes:
      ${VARIABLE:-/dev/null}:${VARIABLE:-/tmp}/app/folder