Remote cache harbor support

[hiddenmarten]

Hello!
Text mostly copied from a Slack conversation.

What existing functionality needs improvement?
I have a question that I couldn’t find in the documentation. Is there a way to add BuildKit parameters for exporting cache to Earthly’s --remote-cache?
In my case, I’m using Harbor and Earthly, and I’m encountering the following issue:

Error: error writing manifest blob: failed commit on ref "sha256:e5cabe1a9719d98fd03ef2e67036faeef57795a2d2ba03fd33c68586e7560188": unexpected status from PUT request to https://harbor.company.team/v2/earthly-cache/test/manifests/cache: 404 Not Found

As a workaround, it might be helpful to try using the flags mentioned in this comment.
I’ve attempted the following:
--remote-cache=harbor.company.team/earthly-cache/test:cache,image-manifest=true,oci-mediatypes=true

However, I encountered an error – invalid reference format Is there another way I can achieve this?

Expected Behavior
I would appreciate it if it would be possible to somehow use Harbor as a remote cache ref.

[hiddenmarten]

I'll open the PR soon.