Changes

common.mk: document some variables to prevent merge-conflicts

Both the DOCKER_COMPOSE_REF and DOCKER_BUILDX_REF versions are frequently
updated, which made it more likely to run into merge conflicts when
pull requests are merged to update either.

Add a comment in between to reduce the probability for such PRs to
conflict.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>