[RFC #0065] - Builder spec

[natalieparellano]

RFC #0065 - Builder spec

Spec:

• https://github.com/buildpacks/spec/tree/distribution/0.3
• Released in distribution API version 0.3

[natalieparellano]

Blocked on #219; once that is complete we can create an RFC to move base image information from platform -> distribution and then (potentially, if the RFC is approved) we can release Distribution 0.3 and then we can close this.