Create a Railway service from a Docker image or GitHub repo via the API

Documented steps

1. Run the serviceCreate mutation with input { projectId, name, source: {...} }.
2. For a Docker image: source: { image: "redis:7-alpine" }.
3. For a GitHub repo: source: { repo: "username/repo-name" }, optionally with input.branch.
4. For an empty service (build command later): omit source or leave it unset.
5. Optionally pass input.variables as an initial env map.
6. Docs: https://docs.railway.com/integrations/api/manage-services