Create a flexible external registry builder (#74)

[dcowley]

The only options for configuring an external DockerImageRegistry are Docker Hub and Google Container Registry. We want the ability to support other registries, like GitLab Container Registry.

This change introduces a flexible builder function to configure an external registry comprised of the following components:

1. Project
2. Namespace (optional)
3. Hostname (optional)

The external registry supports the following image naming convention:

<registry URL>/<namespace>/<project>

For example, these are all supported names for container images:

com.registry.example/mynamespace/myproject

com.registry.example/myproject

mynamespace/myproject