chore: eliminate dependency on docker

[shoenig]

I think we can cut out the transitive dependency on docker if we fixup nomad's pluginutils package

# github.com/docker/distribution
github.com/hashicorp/nomad-driver-podman
github.com/hashicorp/nomad/helper/pluginutils/hclutils
github.com/hashicorp/nomad/helper/pluginutils/hclutils.test
github.com/hashicorp/nomad/drivers/docker
github.com/docker/docker/registry
github.com/docker/distribution/registry/client/auth
github.com/docker/distribution/registry/client
github.com/docker/distribution

Closes #243