Add imageDigests to output to support multi-arch builds on native runners

[jasondamour]

• Added imageDigests output to action
• Added docs on how to utilize imageDigests to do multi-arch builds on native runners in Github Actions

[jasondamour]

@microsoft-github-policy-service agree

[jasondamour]

CC @stuartleeks

[rjstone]

This is also needed for actions/attest-build-provenance

[jasondamour]

Ok I'm running into a little issue. If platform is specified as an input, then the image is output as type=oci,dest=/tmp/output.tar, and can't be found by docker inspect to get the digest.

Would anything break if I moved loading the tar from the runPost to the main script? Will try it out, but warn me of potential pitfalls pls

[jasondamour]

Ok, this is ready for review again. I have a sample workflow successfully working end-to-end (unfortunately in a private repo). The key change I made was coping the oci tar to docker at the end of the build, in addition to the beginning of the post-run step. I need extra review on whether thats ok, and whether I can safely remove the copy from the start of the post-run step.

[jasondamour]

@chrmarti Can you help this?