Django's URLValidator prevents talking to hosts without a domain

But wait, isn't `localhost` a domainless hostname? That works right?

Correct, but `localhost` is a hard-coded exception in the regex jungle of Django URL validation. A metadata URL like `http://django:8000/saml/metadata/` will not work, and cause a subsequent `KeyError` in `SPCreateForm`.

These kinds of hostnames actually come up quite a bit. When running multiple related containers using docker-compose each container (by default) gets a hostname from its service name. So your app, IdP, database, and gateway would each communicate with each other with hostnames like `django`, `idp`, `postgres`, and `nginx`.

Thankfully there's a workaround in docker-compose which involves manually aliasing hosts to `whatever.local` in your network yaml. But it would be nicer to fix it here, as this will come up in practically every Docker deployment.

My suggested solution would be to just turn the `metadata_url` field into a CharField. My dev tools don't need no fancy validation. If that's good with you @tymees I'll include this change in the Dockerfile PR.