RFC 138: Add builder to project descriptor

[dfreilich]

Implement buildpacks/rfcs#138

Summary

Add a builder key to the project.toml file, under build

Since a builder can be selected multiple ways, the following defines the order of precedence when selecting a builder:

1. --builder flag (highest precedence)
2. project.toml → uild.builder
3. configured builder (set via pack config default-builder)

project.toml spec

[project]
id = "<string>"

[build]
builder = "<string>" # optional

[jromero]

We might have missed a prerequisite here. This should be dependant on project descriptor spec which hasn't been created.

See buildpacks/spec#215

Overall, I don't think it's a big deal given the minimal impact of an extra field that is being respected but something to look out for in the future for more impactful changes. For example, reverse domain tables #1127. In those cases, we'd definitely would want to look at api field for the project descriptor spec version.

[sambhav]

There is a pending PR for the spec changes buildpacks/spec#217

Waiting for that to be reviewed/merged.

[dfreilich]

I would argue that it isn't truly a pre-requisite. We can support features that aren't specified; the project descriptor spec is meant to allow for modification and further features.

[fkaleo]

Documentation work remains: buildpacks/docs#345