Add podman support

[MrHedmad]

By editing the new field in kerblam.toml one can swap the docker daemon with podman:

[execution]
backend = "podman" # or "docker", the default

Other backend strings are rejected by serde upon reading the toml. Since podman is essentially identical to docker, the change was just to swap where we use docker to using podman.

TODO

Before merging, tick all of these boxes:

• cargo check passes without errors or warnings.
• @all-contributors is made aware of this PR.