add optional outputs override

[jgreat]

• add docker build-push action raw output override option
• add docker image digest as output
• add action to merge various docker image digests into a manifest (tag) for multi-arch support.
• add simple helper action for getting the "short" SHA tag of a release.

This is improvements to gh-actions for doing multi-arch builds.

Ends up the process is more complex than I was hoping. If you want to do multi-arch with discrete builders (not QEMU or cross compile) you need to first push the images without tags (just the sha256 digest), then you take all the digests and create a manifest that refences the digest and push that up do docker with a tag.

This improves the docker action to allow a raw output override so we can push the bare images and adds a docker-merge-digests action to create the manifest and push up the tag.