Publish Docker image to GHCR on main builds

Add a Docker workflow that builds on PRs and pushes to
ghcr.io/chewbaccalakis/rfid-database on main (latest + sha) and v* tags.
Drop the redundant docker-build job from CI.

.github/workflows/ci.yml

@@ -22,11 +22,3 @@ jobs:
       - run: npm run build
         env:
           AUTH_SECRET: ci-build-secret-at-least-32-characters
-
-  docker:
-    name: docker-build
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - name: Build production image
-        run: docker build -t rfid-database:ci .