Improve Docker image domain resolution

[ruksi]

The logic should be something on the lines of:

TL;DR: The hostname must contain a . DNS separator, a : port separator, or the value localhost before the first /. Otherwise the code assumes you want the default registry, Docker Hub. [docker.io]

Ref: https://stackoverflow.com/a/37867949

As an extra detail, the laituri package should expose the Docker image name parsing as a function in such a way that at least the domain could be resolved when used (other parts and the exact structure are left to the implementer.)

Might also consider the term domain as it can also contain the :port, registry_address maybe?

[akx]

Fixed by #19, released in v0.3.0.