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 default folder.

That is, I want to do something like (pseudocode):

services:    my_awesome_service:        volumes:       if ${VARIABLE} => ${VARIABLE}:/app/folder 

Are such conditional statements definable in a docker-compose file?

The only way I see to make this possible is to first define a base docker-compose file, which does not have the volume mount, and the call on a second docker-compose file only if the $VARIABLE is defined. This is fine for a single or few conditions, but gets nasty if there are many.

Any solution?

Answer 1

Poor man's solution:

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

Or:

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