[feature] Please consider adding a soft-fail flag

Reopened here per @crazy-max request

Original issue: https://github.com/docker/build-push-action/issues/462

Troubleshooting

So I've been working on multi-platform builds as I find time, and what I'm finding is that there are some inconsistencies on shipped packages for say Ubuntu on different architectures. If I simply specify all the platforms I want to build, say linux/amd64,linux/386,linux/arm64,linux/arm/v7,linux/arm/v6,linux/s390x,linux/ppc64le, it would be nice to be able to set a soft-fail where the successful platforms continue on and I get error messages about the platforms that failed.

This makes it much easier to triage which platforms (more than 1) failed and why. Else, I have to manually step through and commit individual platforms to debug what went wrong

Behaviour

See above

Steps to reproduce this issue

1. Add large number of platforms : linux/amd64,linux/386,linux/arm64,linux/arm/v7,linux/arm/v6,linux/s390x,linux/ppc64le
2. If any fail, the entire workflow fails

Expected behaviour

Enable the ability to set a soft-fail flag so that successful builds continue on and does not break the entire build

Actual behaviour

See above.

Configuration

See above.

Logs

See above.

The issue names no implementation files, tests, or entry points; begin by tracing BuildKit's multi-platform build failure handling and review the linked original issue. Done means a soft-fail option is defined so successful platforms continue while failed platforms report their errors without failing the entire workflow.