support `railway up` with dockerfile for docker image services (builder override)

[yigitkonur]

problem

when a service is created with `--image`, subsequent `railway up` source deployments are ignored — the service always pulls the original image. there's no way to switch a service from image-based to source-based deployment via cli.

additionally, `railway.json` with `"builder": "DOCKERFILE"` is not respected by railpack. the dockerfile is ignored even when explicitly configured:

{
  "build": {
    "builder": "DOCKERFILE",
    "dockerfilePath": "Dockerfile"
  }
}

railpack still auto-detects python/node and fails with "no start command detected" despite a valid dockerfile being present.

real-world scenario

deploying a litellm proxy: needed a custom dockerfile that copies a config.yaml into the container. railpack kept detecting the `.yaml` file as python config and ignoring the dockerfile entirely. had to work around by creating a `main.py` shim and using `requirements.txt` instead of the dockerfile.

expected behavior

1. `railway.json` `builder: DOCKERFILE` should override railpack detection
2. `railway up` should work for services originally created with `--image`
3. `railpack.toml` start command should be respected when dockerfile isn't used

related

the `railpack.toml` `[start] cmd = "..."` also wasn't picked up during testing, suggesting the config file discovery may have issues.