Fix frontend-to-backend networking in Docker

Frontend in Docker container can't reach localhost:8000 on the host.
Implemented Docker networking fix using host.docker.internal.

Changes:
  • Updated frontend/src/App.tsx to use host.docker.internal:8000 in production
    - Automatically switches between localhost:8000 (dev) and host.docker.internal:8000 (Docker)
  • Added frontend/.env.example with API configuration
  • Updated docker-compose.yml frontend environment:
    - Added VITE_API_URL=http://host.docker.internal:8000
    - Added comments explaining the networking approach
  • Enhanced DOCKER_QUICK_START.md troubleshooting section
    - Added detailed networking explanation
    - Instructions for verifying host.docker.internal works
    - Linux-specific workarounds

How it works:
  • Docker Desktop automatically maps host.docker.internal to localhost on the host machine
  • Frontend (running in Docker) uses host.docker.internal:8000
  • API & Consumer (running on host) listen on localhost:8000
  • Both reference the same service, just from different network contexts

Workflow remains:
  Terminal 1: docker compose up (infra + frontend in Docker)
  Terminal 2: uvicorn api.main:app --reload (your host machine)
  Terminal 3: cd consumer && python pipeline.py (your host machine)

Frontend automatically reaches API at http://host.docker.internal:8000
without any manual configuration needed.

Co-authored-by: Copilot <[email protected]>

DOCKER_QUICK_START.md

@@ -38,6 +38,8 @@ This starts:
 - ✅ Redis
 - ✅ React Frontend (auto-built and running)
 
+**Important**: The frontend is configured to reach the API at `http://host.docker.internal:8000` (special Docker networking that reaches your host machine from inside the container).
+
 ### Step 2: Start API (New Terminal)
 
 ```bash
@@ -164,20 +166,42 @@ docker compose up --build frontend
 
 ### Frontend can't connect to API
 ```bash
-# Make sure API is running
+# Make sure API is running on host at port 8000
 curl http://localhost:8000/health
 # Should return: {"status": "healthy", ...}
+
+# If API is running but frontend still fails:
+# The frontend uses host.docker.internal:8000 to reach the host
+# This is a Docker Desktop feature that maps to localhost on the host
+
+# Verify it's working with:
+docker exec $(docker ps -q -f name=payguard-frontend) \
+  wget --spider -q http://host.docker.internal:8000
+# Should work without error
 ```
 
 ### Port 3000 already in use
 ```bash
 # Find and kill process
-kill -9 $(lsof -t -i :3000)
+lsof -t -i :3000
 
 # Or use different port in docker-compose.yml
 # Change: "3000:3000" to "3001:3000"
 ```
 
+### "host.docker.internal" not recognized (Linux only)
+On Linux, `host.docker.internal` may not work. Instead:
+```bash
+# Option 1: Pass host IP to container
+docker compose up --build
+# Then edit docker-compose.yml frontend environment:
+# VITE_API_URL=http://172.17.0.1:8000  (default Docker gateway)
+
+# Option 2: Use network_mode: host (shares host network)
+# Add to frontend service: network_mode: "host"
+# Then remove ports, depends_on conditions
+```
+
 ## Full System Architecture
 
 ```

docker-compose.yml

@@ -67,7 +67,11 @@ services:
       retries: 5
     environment:
       - NODE_ENV=production
+      - VITE_API_URL=http://host.docker.internal:8000
     restart: unless-stopped
+    # NOTE: Frontend connects to API running on host machine via host.docker.internal
+    # This allows docker compose up to start infrastructure + frontend,
+    # while API and Consumer run locally on the same machine
 
 volumes:
   redis_data:

frontend/.env.example

@@ -0,0 +1,10 @@
+# Frontend environment configuration
+
+# API Base URL - overrides the default in App.tsx
+# Default behavior:
+#   - Development (npm run dev): http://localhost:8000
+#   - Production (Docker): http://host.docker.internal:8000
+# VITE_API_URL=http://localhost:8000
+
+# Node environment (set by docker-compose.yml)
+NODE_ENV=production

frontend/src/App.tsx

@@ -28,7 +28,10 @@ interface Result {
   timestamp: string;
 }
 
-const API_BASE_URL = 'http://localhost:8000';
+// Use host.docker.internal when running in Docker, localhost when running locally
+const API_BASE_URL = process.env.NODE_ENV === 'production' 
+  ? 'http://host.docker.internal:8000'
+  : 'http://localhost:8000';
 
 const App: React.FC = () => {
   const [auth, setAuth] = useState<AuthState>({