Railway auto-detects Dockerfiles at repo root and uses them for ALL
services, even those set to NIXPACKS. This caused all seed services
(ais-relay, seed-gpsjam, etc.) to build nginx-only containers with
no node binary, breaking every Railway service.
Move Dockerfile and related files to docker/ subdirectory. Railway
only checks the repo root for Dockerfiles, so this prevents
accidental detection. GHA workflow updated with explicit file: path.
