bake: deprecate x-bake compose extension

[crazy-max]

#721 was introduced to allow the use of additional fields that were not supported by the official specification at the time. Now that the Compose specification for build is ahead, we can deprecate this extension and eventually remove it in a future release.

Note that no-cache-filter and output are not yet supported by the official specification.

[glours]

👍

[tonistiigi]

I don't get this. Is this expecting that bake will never add new fields in the definition? Plus, there are many fields in bake definition not supported in compose build block.

[crazy-max]

Is this expecting that bake will never add new fields in the definition? Plus, there are many fields in bake definition not supported in compose build block.

No it just means that the official compose specification needs to add new fields to build block. x-bake was meaningful when there was no parity between bake fields and the official spec at the time.

Only missing fields from the official spec that are supported today in x-bake extension are no-cache-filter and output. We can postpone deprecation until these fields made into the official spec.