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>

README.md

@@ -72,6 +72,17 @@ Replace `username` with your actual Git username and `your-repo` with your repos
 
 The application will be available at: `https://your-subdomain.startanaicompany.com`
 
+**Important: Enable Git Commit Tracking**
+
+To see the git commit ID in the `/health` endpoint, you must enable "Include Source Commit in Build" in your application's General settings in Coolify:
+
+1. Go to your application in Coolify dashboard
+2. Navigate to General settings
+3. Enable "Include Source Commit in Build"
+4. Redeploy your application
+
+**Note:** This will invalidate Docker build cache on every commit (since the commit hash changes), but it allows you to track which version is deployed.
+
 ### 5. **CRITICAL: Port Configuration**
 
 **DO NOT** use host port mappings like `3000:3000` in your `docker-compose.yml` file!
@@ -432,11 +443,13 @@ Health check endpoint that verifies connections to PostgreSQL and Redis, and ret
 ```
 
 **Version Tracking:**
-- `git_commit`: Short Git commit SHA (7 characters) of the deployed code, auto-detected during build using `git rev-parse --short HEAD`
+- `git_commit`: Short Git commit SHA (7 characters) of the deployed code, provided by Coolify's `SOURCE_COMMIT` environment variable (first 7 characters). Returns "unknown" if "Include Source Commit in Build" is not enabled in Coolify settings.
 - `version`: Application version (defaults to 1.0.0, can be set via APP_VERSION env var)
 
 This allows AI agents and monitoring tools to verify which version is currently running. The short commit ID is industry standard for deployment tracking, providing a balance between uniqueness and readability.
 
+**To enable commit tracking:** Go to your application's General settings in Coolify and enable "Include Source Commit in Build", then redeploy.
+
 ---
 
 ## Troubleshooting