Try using Depot

[mnapoli]

This is an attempt at using https://depot.dev to accelerate builds with caches.

[mnapoli]

This is a complete success!

Without Depot:

• x86, without cache: ? (don't have a base rn)
• x86, with cache: 10m
• arm, without cache: 2h30 - 3h (!!)
• arm, with cache: no reference, probably 10m

Important note: with full caching, I managed to get done to 2-3 minutes, but that wasn't stable as the cache was too big and failed most of the time. That's why I cached less things and got to 10min, but even then that wasn't stable enough to work all the time.

With Depot:

• x86, without cache: 10m
• x86, with cache: 1-2m
• arm, without cache: 9m
• arm, with cache: 1-2m
  To be concrete, I upgraded the PHP versions and the PR was green in 10 minutes. That's the end-user experience and I'm very happy with it!

Also note that using Depot is completely optional, and used only in CI by default. Locally it will build with docker buildx bake by default, so we are closing the build for users.