[Singularity] Support for additional image protocols?

[hexylena]

Hi y'all,

In wdl 1.1, the definition of the container argument states

The format of a container URI string is protocol://location, where protocol is one of the protocols supported by the execution engine. Execution engines must, at a minimum, support the docker:// protocol, and if no protocol is specified, it is assumed to be docker://.

For my use case (apptainer, from a 'docker export' tarball), it would be helpful if the protocol was not hardcoded in miniwdl, and instead prepended when no :// was present in the container name. Then I could make use of docker-archive://path/to/image.tar to load the container export and build the .sif on demand.

I recognise this is a bit of an obscure use case, but, it would save me significant time of creating a public repo (not really desirable for this case), and pushing my image there. For my use case, I cannot run docker on the host where miniwdl is running, thus I need to make use of an alternative protocol or mechanism.

I'm not sure what the correct protocol to load a SIF directly into the singularity backend would be (file:// ?) but if that could be supported at the same time (by permitting any protocol and letting the backend determine validity), then that would be really useful.