Add a Docker image and compose file for the web proxy

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,18 @@
+# The browser version: web app and proxy in one image.
+#   docker compose up -d --build
+FROM node:22-slim AS build
+WORKDIR /src
+COPY package.json package-lock.json ./
+# No install scripts: the Electron download they would trigger is only needed for the desktop app
+RUN npm ci --ignore-scripts
+COPY . .
+RUN npm run build:web
+
+FROM node:22-slim
+WORKDIR /app
+COPY --from=build /src/dist-web ./dist-web
+COPY --from=build /src/dist-proxy ./dist-proxy
+USER node
+ENV MUMH5_PORT=8080
+EXPOSE 8080/tcp 3478/udp
+CMD ["node", "dist-proxy/proxy.mjs"]