Consider fully qualifying image names in docker-compose files

[metasoarous]

As @willcohen pointed out in #1135, he made a pass of fully qualifying image names due to a requirement that podman had at the time that this be done. Podman has since rolled back this requirement in order to maintain broader compatibility with extant docker-compose files, and I think for now we're going to merge #1135 without the qualification, since I don't have a full grasp of all the implications there. However, at some point, it would be a good idea to consider this more carefully.