Add monorepo support for nixpacks

[ohkimur]

What problem will this feature address?

Currently, although it's possible to set the build path for a specific directory, it's not possible to configure nixpacks to run an app from another directory in a monorepo rather than root.

Describe the solution you'd like

It'll be nice to have options to configure nixpacks like railway: https://docs.railway.app/guides/monorepo

Describe alternatives you've considered

Creating my own Dockerfile or adding start command in the root of the monorepo for my app, but... that's not ideal because I have multiple apps, which is the whole point of a monorepo.

Additional context

No response