feat(docker): use pre-built GHCR image in docker-compose by default (#25)

docker-compose.yml now references ghcr.io/ulsklyc/oikos:latest so users
can start the app with a single 'docker compose up' without cloning or
building locally. The build: . entry is retained for contributors who
want to build from source with --build.

README Quick Start updated to document both the no-clone path (curl
docker-compose + .env.example) and the build-from-source path.

docker-compose.yml

@@ -1,6 +1,7 @@
 services:
   oikos:
-    build: .
+    image: ghcr.io/ulsklyc/oikos:latest
+    build: .                          # optional: use --build to build locally instead
     container_name: oikos
     restart: unless-stopped
     ports:
@@ -12,7 +13,7 @@ services:
     environment:
       - NODE_ENV=production
       - DB_PATH=/data/oikos.db
-      # Auf false setzen wenn kein HTTPS/Reverse-Proxy (direkter HTTP-Zugriff)
+      # Set to false when not using HTTPS/reverse proxy (direct HTTP access)
       - SESSION_SECURE=false
     healthcheck:
       test: ["CMD", "node", "-e", "require('http').get('http://localhost:3000/health', r => process.exit(r.statusCode === 200 ? 0 : 1))"]