Allow custom registry + auth

[karimsa]

Hi, there! This project looks pretty cool and I'd like to be able to use it. However, I currently use a private registry that isn't Docker Hub and would like to be able to override the default URI + auth info. I'm a bit busy right now but might be able to open a PR for this in a few days.

[jakowenko]

Thanks for checking this out. I can definitely look into implementing that. Are you running a private registry similar to https://hub.docker.com/_/registry?

I don’t have a lot of experience with them, so will need to set it up locally to test with.

[karimsa]

I'm not running it myself, I'm using a provided solution. You can do this via Quay.io, Heroku, DigitalOcean, or AWS ECR.