feat: Docker Compose support (#1729)

Adds support for renovating Docker Compose files (e.g. `docker-compose.yml`). Functionality is essentially the same as the existing `Dockerfile` capabilities, so config for `docker` is shared with `docker-compose` but may also be overridden.

Merging as disabled by default - will wait for some opt-in testing before turning it on by default.

Closes #832