fix: use environment variables for TS_AUTHKEY and TS_LOGIN_SERVER

- docker-compose.yml: TS_AUTHKEY and TS_LOGIN_SERVER now read from environment
- README.md: updated Tailscale configuration instructions to reference env vars instead of editing the file
- Added .gitignore to prevent committing secrets or Tailscale state

This protects reusable test auth keys and custom coordination servers from being committed to any ts-* repo.

docker-compose.yml

@@ -4,7 +4,8 @@ services:
     hostname: {{service}}
     container_name: {{service}}-ts
     environment:
-      - TS_AUTHKEY={{YOUR_TAILSCALE_AUTHKEY}}
+      - TS_AUTHKEY=${TS_AUTHKEY}
+      - TS_LOGIN_SERVER=${TS_LOGIN_SERVER}
       - TS_STATE_DIR=/var/lib/tailscale
       - TS_SERVE_CONFIG=/config/serve.json
     volumes: