fix(docker): secure environment files in Docker setup

Dockerfile

@@ -1,4 +1,23 @@
 FROM nginx:1.25.3-alpine
 
-WORKDIR /var/www/codesandbox
-COPY www ./
+# Nginx config: serves /usr/share/nginx/html and denies dotfiles (e.g. /.env).
+COPY .github/nginx/default.conf /etc/nginx/conf.d/default.conf
+
+# Static assets go under the document root.
+COPY www /usr/share/nginx/html
+
+# Runtime env template + generator live OUTSIDE the document root so they are
+# never reachable over HTTP. `env.sh` reads `.env` (and any overriding real
+# environment variables) and writes `env-config.js` into the served static dir.
+COPY .env /app/.env
+COPY env.sh /app/env.sh
+RUN chmod +x /app/env.sh
+
+# Defense-in-depth: make absolutely sure no stale copy of .env slipped into
+# the web root from a previous build.
+RUN rm -f /usr/share/nginx/html/.env /usr/share/nginx/html/env.sh
+
+WORKDIR /app
+
+# Generate env-config.js at container start, then hand off to nginx (PID 1).
+CMD ["/bin/sh", "-c", "/app/env.sh /usr/share/nginx/html/static/js/env-config.js && exec nginx -g 'daemon off;'"]

packages/app/scripts/copy-assets.js

@@ -24,14 +24,13 @@ const assets = [
     from: 'packages/app/public',
     to: '',
   },
-  {
-    from: '.env',
-    to: '.env',
-  },
-  {
-    from: 'env.sh',
-    to: 'env.sh',
-  },
+  // NOTE: `.env` and `env.sh` are intentionally NOT copied into `www/`.
+  // `www/` is served publicly by nginx, so anything placed here is reachable
+  // from the internet (e.g. GET /.env would leak secrets). `env.sh` reads
+  // `.env` at container startup to generate `static/js/env-config.js`, which
+  // is the only file the browser needs. The Dockerfile places `.env` and
+  // `env.sh` under `/app` (outside the document root) and runs `env.sh` from
+  // there on container start.
 ].filter(Boolean);
 
 const rootPath = path.resolve(__dirname, '../../..');