Draft: Add a caddy reverse proxy configuration

Please avoid merging the base branch into your feature branch. We are working with rebases and those merged tend to cause trouble. For further questions, join us at Matrix: https://matrix.to/#/#funkwhale-dev:matrix.org

If your contribution is fixing an issue by a small change, please consider a merge into stable by using it as target branch.

Related issue: !1384 (closed)

This Merge Request includes:

• Tests
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)