Build and publish a multi-arch server image to GHCR

[bharat]

Thanks for keeping this project moving. The active development here is what made me want to run your server build instead of the older PyPI release, and a ready-made container image was the missing piece, so here is the workflow I built for it.

What this does

• Adds a standalone root Dockerfile that installs the package from the checked-out source and starts the server. The server console script is not installed by packaging, so the entrypoint invokes concord232.main directly.
• Adds a GitHub Actions workflow that builds linux/amd64 and linux/arm64 images and publishes them to ghcr.io/<owner>/concord232 on pushes to main and on v* tags. It authenticates with the built-in GITHUB_TOKEN, so no secrets or account setup are needed on your side.
• Documents the image in the README.

Tags: the branch name on branch pushes, the version plus latest on v* release tags, and the commit sha for pinning.

Notes

• The workflow uses github.repository, so it runs unchanged on any fork. The extra docker-image branch trigger is there so forks can build without touching main; happy to drop it, rename the image, or reshape the tags if you prefer.
• Publish path verified end to end: this branch built and published a multi-arch image on my fork, https://github.com/bharat/concord232/pkgs/container/concord232, which I am switching my own Concord 4 deployment to.
• The ghcr package created on first publish defaults to private; making it public is a one-time toggle in the package settings.