Feature Request: Support for alternate container registries

[lpiep]

Right now the run command has docker:// hardcoded as the registry to search, which seems to mean I can't use containers in other places (such as ghcr.io).

> run("oras://ghcr.io/kaufman-lab/geospatial_plus:4.1.0", "echo", c("hello"))
Error: FATAL:   Unable to handle docker://oras://ghcr.io/kaufman-lab/geospatial_plus:4.1.0 uri: failed to get checksum for docker://oras://ghcr.io/kaufman-lab/geospatial_plus:4.1.0: unable to parse image name docker://oras://ghcr.io/kaufman-lab/geospatial_plus:4.1.0: invalid reference format

It would be nice to either have the run function parse the container ID for the oras://ghcr.io (or whatever else) prefix, or to be able to provide the registry as a separate argument. Alternatively, I may be misunderstanding how to specify the container id argument.

I'm happy to help with a PR.