Specification for CNB_STACK_ID changed when it was deprecated in Platform API 0.12

[edmorley]

Platform API 0.11 says:

Build Image

The platform MUST ensure that:

• ...
• The image config's Env field has the environment variable CNB_STACK_ID set to the stack ID.

(https://github.com/buildpacks/spec/blob/platform/v0.11/platform.md#build-image)

Under "run image" there is no mention of CNB_STACK_ID - ie: the env var only needs to be set in the build image.

However, when stacks were deprecated in Platform API 0.12, and the spec updated in #335, the specification was changed to say the CNB_STACK_ID env var should be set for both the run and build image:

For compatibility with older platforms and older buildpacks, base image authors SHOULD ensure for build images and run images:

• The image config's Env field has the environment variable CNB_STACK_ID set to the stack ID.

(https://github.com/buildpacks/spec/blob/platform/v0.12/platform.md#iobuildpacksstack-labels)

I'm presuming this change was inadvertent?

[edmorley]

I've opened #397 for this. (We'll also want to backport to the older branches)