[RFC #0096] validate buildpacks' targets to based images

[buildpack-bot]

Today, mixin validation is done by pack both at the time of builder creation and at build time.

For Platform API 0.12 and greater, we should replace mixin validation with target data validation. Namely,

In buildpack.toml:

[[targets]]
os = "linux"
arch = "x86_64"
[[targets.distributions]]
name = "ubuntu"
versions = ["18.04"]

In image config:

• os
• architecture
• variant (if specified)
• io.buildpacks.distribution.name label (if specified)
• io.buildpacks.distribution.version label (if specified)

Build-time base image config should match runtime base image config. Buildpack.toml should match both of these.

If an (older) buildpack does not declare target data in buildpack.toml BUT it sets stacks.id = "io.buildpacks.stacks.bionic, pack should assume the following values:

[[targets]]
os = "linux"
arch = "x86_64"
[[targets.distributions]]
name = "ubuntu"
versions = ["18.04"]

If the build-time or runtime base is missing target data but the image contains the label io.buildpacks.stack.id with value io.buildpacks.stacks.bionic, pack should assume the following values:

• run-image.target.os = "linux"
• run-image.target.arch = "x86_64"
• run-image.target.distribution.name = "ubuntu"
• run-image.target.distribution.version = "18.04"

[natalieparellano]

This validation is done in the lifecycle here: https://github.com/buildpacks/lifecycle/blob/2b6ddbb7af1d7ffe11803c5f4c3eaa677f889f0e/detector.go#L235-L245