chore: consolidate .env.example to docker folder only

- Remove root .env.example (now only in docker/ folder)
- Update .gitignore to not track root .env.example
- Keep docker/.env.example as single source of truth

.gitignore

@@ -36,8 +36,6 @@ yarn-error.log*
 .env.development.local
 .env.test.local
 .env.production.local
-# Keep .env.example tracked
-!.env.example
 
 # vercel
 .vercel