Support Railpack as a deployment strategy

### Description

[Railpack][] is the [formal successor]() to Nixpacks. Kubero should support Railpack as an alternative to Nixpacks.

[Railpack]:

### Possible solution

Kubero should support Railpack as a deployment strategy.

### Alternatives

_No response_

### Additional information

See for the release announcement.

Start by comparing Kubero's existing Nixpacks deployment strategy with Railpack's documentation and release announcement. Identify the deployment-strategy entry points and any related tests, then verify that Railpack can be selected as an alternative and completes a deployment successfully.

Written by the indexing model from the issue text.