Fix deployment: remove build context from docker-compose.yml

Production servers pull the pre-built image from ghcr.io — having
build: . caused docker compose up to fail when no Dockerfile was present.

Moved build: . to docker-compose.override.yml (gitignored) which Docker
Compose automatically merges locally for dev builds. Production deployments
only need docker-compose.yml and .env.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -1,6 +1,5 @@
 services:
   app:
-    build: .
     image: ghcr.io/djsplice07/squares_game:latest
     network_mode: host
     env_file: .env