Support multiple providers

[coffee-cup]

You should be able to specify that you want to use multiple providers. Each provider should run in order and contribute to the deploy layers. The first (or last?) provider can provide the start command.

I don't think this should happen by default, but it should be easy to specify with a config env var or in the railpack.json.

I have a branch that has it mostly working, main...jr/multiple-provider-support.

Reasoning

You can already specify multiple languages by setting the RAILPACK_PACKAGES variable (e.g. RAILPACK_PACKAGES=python go node). However, you lose out on the automatic version detection and layer optimizations that happens for each provider. You should be able to specify that you want Node and Python and have the correct versions of everything installed.

[iloveitaly]

Very much agree here. The analog to this is the hacky setup I have now where I:

1. Build the javascript app (SPA)
2. Start a container, then extract the built assets, put into a folder that the python app uses
3. Build the python app
4. Deploy the python app

Will think on this once most of the obvious nixpacks-compat stuff is done.