926 migrate beta deploy to use ghcr instead of docker hub 2

[augustjohnson]

This PR updates the CI/CD pipeline to publish the staging/beta image to GitHub Container Registry (GHCR) in addition to Docker Hub, and adds two workflow quality-of-life improvements.

1. The staging branch now pushes to both eepmoody/open5e-api:staging (Docker Hub) and ghcr.io/open5e/open5e-api:beta (GHCR). This will be a longer-term goal, but for now both locations are targets.
2. Added paths-ignore to skip CI on doc/script-only changes, and added a concurrency group so in-progress deploys to the same branch are cancelled when a newer push arrives.
3. .dockerignore: This branch adds a root-level .dockerignore to prevent .git, caches, local env files, markdown docs, and other non-essential files from being sent to the Docker build context. data/, source dirs, pyproject.toml, uv.lock, and manage.py are intentionally kept as they are required by the multi-stage build.

[augustjohnson]

This is ready for review!
The bonus changes here at the end had a few non-functional changes. The dockerfile should be significantly smaller now (no /data directory), and quicksetup should run faster due to a couple of tweaks without workflow impacts.