ci: add Dockerfile and Gitea Actions workflow for automated image builds

- Dockerfile based on node:18-alpine
- Gitea Actions workflow triggers on push to release/* branches
- Builds and pushes to Gitea container registry with version tags

Dockerfile

@@ -0,0 +1,17 @@
+FROM node:18-alpine
+
+WORKDIR /app
+
+# Copy package files and install dependencies
+COPY package.json package-lock.json ./
+RUN npm ci --omit=dev
+
+# Copy application source
+COPY server/ ./server/
+COPY public/ ./public/
+
+EXPOSE 3001
+
+ENV NODE_ENV=production
+
+CMD ["node", "server/index.js"]