feat(backend): configure Dockerfile and express server to build and serve mobile app at /mobile on Cloud Run

backend/.env.test

@@ -0,0 +1,14 @@
+PORT=8080
+NODE_ENV=development
+API_PREFIX=/api
+CORS_ORIGIN=*
+LOG_LEVEL=debug
+PROJECT_ID=test-project
+DATABASE_URL=libsql://test-db.turso.io
+DATABASE_AUTH_TOKEN=test-token
+S3_ENDPOINT=https://t3.storage.dev
+S3_BUCKET=test-bucket
+S3_PREFIX=test-prefix
+S3_ACCESS_KEY_ID=test-key
+S3_SECRET_ACCESS_KEY=test-secret
+S3_SESSION_TOKEN=