Fix: Use Coolify's SOURCE_COMMIT instead of git detection

Research findings:
- Coolify provides SOURCE_COMMIT environment variable with git commit hash
- Must enable "Include Source Commit in Build" in Coolify General settings
- Disabled by default to preserve Docker build cache
- Coolify automatically passes SOURCE_COMMIT as build arg

Changes:
1. Dockerfile: Use SOURCE_COMMIT build arg instead of GIT_COMMIT
2. Dockerfile: Remove git installation and detection logic
3. Dockerfile: Extract first 7 chars of SOURCE_COMMIT for short hash
4. docker-compose.yml: Pass SOURCE_COMMIT instead of GIT_COMMIT
5. docker-compose.yml: Remove GIT_COMMIT from env (set in Dockerfile)
6. README: Add instructions to enable "Include Source Commit in Build"
7. README: Update version tracking documentation

How it works:
- Coolify passes SOURCE_COMMIT to docker build
- Dockerfile takes first 7 characters: ${SOURCE_COMMIT:0:7}
- Sets as GIT_COMMIT env var for runtime
- Health endpoint returns the short commit ID

Note: Returns "unknown" if SOURCE_COMMIT tracking is not enabled.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docker-compose.yml

@@ -5,7 +5,7 @@ services:
     build:
       context: .
       args:
-        GIT_COMMIT: ${GIT_COMMIT:-unknown}
+        SOURCE_COMMIT: ${SOURCE_COMMIT:-unknown}
         APP_VERSION: ${APP_VERSION:-1.0.0}
     restart: unless-stopped
     environment:
@@ -18,7 +18,6 @@ services:
       - POSTGRES_DB=template_db
       - REDIS_HOST=redis
       - REDIS_PORT=6379
-      - GIT_COMMIT=${GIT_COMMIT:-unknown}
       - APP_VERSION=${APP_VERSION:-1.0.0}
     depends_on:
       postgres: