ci: revert to Docker image build + registry push (no deploy)

Drop the LXC deploy step. Pipeline now stops at publishing the image
to the Gitea registry; deployment is handled out of band.

Restored:
- Dockerfile (multi-stage: curl-builder + quarkus-micro-image:2.0,
  generic via build-output/*-runner wildcard, COPY --chown=1001:1001)
- compose.yaml (one-shot install of the published image)
- .dockerignore (excludes build-output/)

CI workflow:
- Installs docker-buildx (needed for COPY --chown)
- Uses docker buildx build
- chmod 775 and echo securerandom happen in the 'Stage binary for Docker'
  step; the final image has no RUN commands
- Tags :latest and :<short-sha>, pushes with retry

No deploy step. Pull the image with docker compose / run it manually.

.dockerignore

@@ -0,0 +1,9 @@
+.git
+target
+node_modules
+data
+.mvn
+*.md
+.gitignore
+.gitea
+build-output