Previously Compass had two separate container setups - one for users who wanted to build their own image, and another for users who just needed one that was already pre-built.

Reorganized Docker setup to use the prebuilt GHCR image for Compass, even for "custom" or local built images. Users who wish to build their own custom image can still do so, using the exact same `Dockerfile` at the root of the project.

There isn't really a need to have two styles of container images, and it also made using local sources for Compass difficult.

Update GitHub Action workflow to point to the new `Dockerfile` location.

Consolidated environment variable handling for Docker (both "prebuilt" and "local" setups use the pre-build style), simplified Compose setup, and updated documentation to reflect the new workflow.