I'm deploying a FastAPI backend and consistently getting this error:

  Error: Invalid value for '--port': '$PORT' is not a valid integer.

What I've tried:

  1. Dockerfile with exec form CMD:

  CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]

  2. Dockerfile with shell form CMD:

  CMD uvicorn app.main:app --host 0.0.0.0 --port ${PORT:-8000}

  3. Dockerfile with explicit sh -c:

  CMD ["sh", "-c", "uvicorn app.main:app --host 0.0.0.0 --port ${PORT:-8000}"]

  4. railway.toml configuration:

  [build]

  builder = "nixpacks"

  [deploy]

  startCommand = "uvicorn app.main:app --host 0.0.0.0 --port $PORT"

  All attempts result in the same error. The build succeeds but the container fails to start.

Project structure:

  backend/

  ├── Dockerfile

  ├── railway.toml

  ├── requirements.txt (fastapi, uvicorn)

  └── app/

      └── main.py

  Is Railway using my Dockerfile or Nixpacks? How can I properly pass the PORT variable to uvicorn?

Deployment logs show:

  - Build succeeds

  - Container starts

  - Immediately fails with PORT error

  - Health check fails repeatedly

  Any help appreciated!