support pushing to ghcr.io registry

Howdy, This is Richard at TAMU. I am investigating the use of container registries. This is because I have learned that Charliecloud team is able to teach the use of registries in the Tutorial that we are trying to schedule.

I was able to push an image to my personal GitLab registry ch-image push hello registry.gitlab.com/rarensu/ch-image-test:hello (success) (success)

However, when I tried the same thing with my personal GitHub registry, it failed. ch-image push hello ghcr.io/rarensu/ch-image-test:hello error: PUT failed: Invalid URL '/v2/rarensu/ch-image-test/blobs/upload/384a86f0-97af-4fa4-900f-bd5698a71979': No schema supplied. Perhaps you meant http:///v2/rarensu/ch-image-test/blobs/upload/384a86f0-97af-4fa4-900f-bd5698a71979?

I turned on verbose so I could see the HTTP actions.

In the GitLab case, the POST action to request an upload blob location got a response that contained a blob location URI. location: https://registry.gitlab.com/v2/rarensu/ch-image-test/blobs/uploads/... This was successfully used as the target of the subsequent PUT action to upload a blob.

However, in the GitHub case, the POST action response contained a blob location that was not in the form of a URI. location: /v2/rarensu/ch-image-test/blobs/upload/... The subsequent PUT action failed because of course, HTTP did not recognize it as a valid URI.

I suspect that this is due to GitHub using a different API specification. I do not know what the solution should be. Maybe complain to GitHub and get them to change their API specification to match GitLab? I wonder how the other container runtimes are handling this.

Richard Lawrence