Allow extensions in build field in docker-compose.yaml schema

[zkulbeda]

Enable support of x-bake extension in json schema.

- What I did
Add patternProperties in latest json schema for build field.

- Description for the changelog
Add support for x- fields in build field in docker-compose.yaml

- A picture of a cute animal (not mandatory but encouraged)

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        /    \   '. __.'
       _|    /_  \ \_\_
jgs   {_\______\-'\__\_\

[thaJeztah]

Thanks! We discussed this change, and decided that instead, we'll allow all additional properties, because stack does not support build, and already ignores it, so we might as well ignore all properties underneath it.

See #4214