Implement ability to load images by default in non-Docker build drivers.

[n-g]

This eases build driver migrations, as it allows aligning the default behavior.

See also https://docs.docker.com/build/drivers/

[n-g]

This could also be a driver-opt maybe?

Thanks! I like this path, as it allows finer granularity when selecting the new behavior (per driver vs per env). From what I can tell, this is the first driver-opt applicable to multiple driver types. I introduced a new feature, but lmk if there is a cleaner modeling path I could employ.

[n-g]

@tonistiigi @crazy-max Thanks for taking a look at this! From what I can tell, I addressed all your suggestions. Is there anything left to do to make this PR ready to merge? Thanks!

[n-g]

Can you squash your commits please?

Also would be nice to have integration tests in https://github.com/docker/buildx/tree/master/tests if you can or can be follow-up

Thanks!

Sure!

I initially did not realize that not all workers are tested by default with ./hack/test. Updated the test now, and validated it with TESTFLAGS="-v --parallel=1 --timeout=30m --run=//worker=docker$" TEST_DOCKERD=1 TESTFLAGS_DOCKER="-v --parallel=1 --timeout=30m" ./hack/test locally.

PTAL

[n-g]

@crazy-max Could you help me re-trigger the validation workflows?

[n-g]

@crazy-max Could you help me re-trigger the validation workflows?

Thanks! Let me know if there is anything else I can help with here 🙏

[n-g]

It needs a rebase with changes introduced recently sorry 😅 See also review, I tried to push changes to your branch but looks like you don't allow edits by maintainers. Here is my branch if you want to pick these changes: master...crazy-max:buildx:default-load-opt otherwise I can carry in another PR if you prefer. Thanks

Thanks! Rebased and applied your suggestions.

From what I can tell, I cannot grant edit rights retroactively. Feel free to ping me if you need any changes.