Add Dockerfile, nginx config and publish CI

Add a multi-stage Dockerfile to build the React app (node:24-alpine, npm ci, npm run build) and serve the compiled /app/dist via nginx:alpine. Include nginx.conf that routes unknown paths to /index.html so React Router works in a SPA. Add a CI workflow (.forgejo/workflows/docker-publish.yml) to build and push the Docker image to Codeberg Container Registry on pushes to main (requires CODEBERG_TOKEN repository secret and tags the image as codeberg.org/${{ github.repository }}:latest).

nginx.conf

@@ -0,0 +1,10 @@
+server {
+    listen 80;
+
+    location / {
+        root /usr/share/nginx/html;
+        index index.html index.htm;
+        # Redirect all unknown paths to index.html to allow React Router to handle the routing
+        try_files $uri $uri/ /index.html;
+    }
+}