feat(nx-container): add support for registry option

[callajd]

I went ahead and put together a small commit to enable a registry option:

• optional, string, off by default;
• when registry is defined, this change simply prefixes each tag with the value of registry and a /, exactly as the docker docs use 'registry'.

All of the existing tests pass but it wasn't clear to me where I would add new tests for this option. Happy to add new tests if you could point me in the right direction.

[gperdomor]

Hi @callajd thank you for you interest in this project and for make this contribution, sadly I can't merge this basically because it's already covered using the tags or images options depending if you use metadata or not... The second reason why I can't merge this it's because we will break the possibility to push to multiples registries which it's also covered with tags or images options...

[callajd]

The problem with the existing solution is that it (whether accidentally or intentionally) shoehorns the registry concept into the distinct concepts of images and tags, which distorts the meanings of images and tags while making valid and common use cases unnecessarily difficult.

• a registry has a collection of repositories
• a repository is a collection of images that have tags

See for example Docker's explanation, or Google Cloud's.

Based on your comments, I've updated my PR to use 'registries' as a string[] instead of 'registry' as a single string, so this new option supports multi-registry deployment. As before, the setting remains optional so existing users can continue using the same approach for multi-registry deployment without issue.