Sign official images with OpenPubKey #561

Tell us about your request
It would be helpful to support OpenPubKey to verify official images from Docker. This could be done in addition to other signing solutions to give users the flexibility to use their own preferred signing solution.

Which service(s) is this request for?
Docker Official Images (DOI).

Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
Verify the authenticity of official images. This can only be done by Docker.

Are you currently working around the issue?
Using images other than DOI or using DOI images without verifying their authenticity.

Additional context
I'll open similar issues for other signing tools.