[RFC #0109] Allow setting environment variables using the build image

[buildpack-bot]

This issue have been automatically created from pull request buildpacks/rfcs#230.

[edmorley]

It seems this should also be added to a Buildpack API milestone too (and not just the Platform 0.11 milestone), since buildpack authors need to be aware of this change as well.

For example, if a buildpack author uses clear-env = true (in order that the buildpack author has more explicit control over env vars passed to any subprocesses launched by the buildpack), then the buildpack author in some cases will want to load the env vars from /cnb/build-config and pass them to subprocesses that need them (similar to what buildpack authors already have to do for platform env vars when using clear-env = true).

[natalieparellano]

@edmorley thanks for bringing this up. I think we got in a weird state here where Platform API 0.11 is released but Buildpack API 0.10 (which describes the build config dir for buildpacks) is not (see: "This applies for all values of clear-env or if clear-env is undefined in buildpack.toml."). For the case that you describe, I don't think the buildpack needs to take any particular action in order to propagate the build-config/<env> to future buildpacks.

I do have a question @AidanDelaney @samj1912 the platform spec says "Operator-defined environment variables MAY be modified by subsequent buildpacks before they are provided to a given buildpack." (it says the same thing for "User-provided environment variables" i.e., <platform>/env). But looking at the lifecycle code that seems not to be the case - we always override the buildpack-provided env with the platform env... is the spec wrong? Does anyone remember the original intention here?

[natalieparellano]

Closed in Platform 0.11 and Buildpack 0.10