fix: pin PocketBase version and improve documentation

- Pin PocketBase image to v0.22.4 instead of latest
- Add healthcheck to docker-compose.yml
- Add environment variables documentation to README.md
- Add development and production environment sections

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/docker-compose.yml

@@ -2,7 +2,7 @@ version: '3.8'
 
 services:
   pocketbase:
-    image: ghcr.io/muchobien/pocketbase:latest
+    image: ghcr.io/muchobien/pocketbase:v0.22.4
     container_name: gamegroup-pb
     ports:
       - "${PB_PORT:-8090}:8090"
@@ -12,3 +12,9 @@ services:
     environment:
       - GO_ENV=production
     restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "wget", "--no-verbose", "--tries=1", "--spider", "http://localhost:8090/api/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 40s