Simplify Makefile

[JosephMarinier]

Description:

• COMPOSE_EXT used to overwrite the -f docker-compose.yml by specifying a second -f docker-compose-gpu.yml, which (fortunately) made docker-compose "forget" about the first file. This undocumented behavior is not straightforward and makes our Makefile confusing. I propose making COMPOSE_EXT only handle the -gpu part of the file name.
• When running docker-compose, a message suggests using docker compose instead.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[gabegma]

Ya, great!