That would be really interesting indeed, but kind of curious how you would overcome the legal issues that prevented them to do so

actions/runner-images#176 (comment)

Hey @ggirard07 thanks for sharing. The idea would be less to make it available in the marketplace, but more to implement the logic, some of the participants in the issue mentioned, to built an image based on the Packer file published in the referenced GitHub repository. So effectively, the consumer would need to build the images themselves.

The main reason this isn't implemented yet is that it turned out that the Packer built is very unstable (failing usually 2/3+ of times) which isn't really a good look. I can only assume this works better when using ImageTemplates as they may have a lot of retries implemented in there. Unfortunately it is currently not possible to use that Packer file directly as a customization step in the Image Template resource. Meh...