introduce cache-to

[ndeloof]

What this PR does / why we need it:
Introduce cache-to in the compose specification, to offer a canonical binding with buildkit support for caching.

cc @CrazyMax @ulyssessouza @tonistiigi

[EricHripko]

Thank you for putting this together - I got a general question to better understand the use case ❓

I can certainly see the value for a shared cache (e.g., on NFS or in the registry) being referenced by Compose via cache_from. What is the use case for specifying cache_to in Compose though? Something like a local path especially seems rather machine-specific and possibly better off left to the tooling (e.g., via some CLI interface or export command).

[ndeloof]

cache_to supports allows to share layers from intermediary images in a multi-stage Dockerfile, which are not pushed with the main image. Also allows to configure cache mechanisms not based on image registry, like github actions cache.

[EricHripko]

Thank you for elaborating! I guess there's image, registry and inline caches that aren't machine-specific, strictly speaking 👍

[EricHripko]

Looks like a solid addition! Got a quick typo and a question inline 💬

[ndeloof]

@EricHripko fixed, can I get your Approval on this one ?

[ulyssessouza]

LGTM

[EricHripko]

Looks good once the typos inline are sorted ✅ Thank you for seeing this through 👍