Support dsub as an alternative container backend

[agitter]

dsub is a command line tool for running commands in Google Cloud inside a container. @nisthapanda is investigating whether we could use it to run containers in SPRAS because that may be one path toward using SPRAS in the restricted NIH All of US platform. The solution might be a further generalization of our run_container utility function.

This is related to #124 and Podman, another possible backend for containers.

[agitter]

Nistha has a branch that successfully runs Omics Integrator 1 using dsub in the All of Us environment: https://github.com/nisthapanda/spras/tree/dsub

We don't have immediate plans to generalize this and merge this back into SPRAS. It relies on user-specific information about the environment and location of storage buckets.

[agitter]

The changes in #132 that make the registry configuration make this easier to support now.

[agitter]

@nisthapanda discovered that dsub requires bash to run containers, which isn't available in Alpine Linux. I'm going to test adding bash to PathLinker to see if that resolves her problem.