Support multiple private docker registries

[mario]

Cloudron supports configuring 1 private support registry and it would be beneficial if it supported multiple.

[Lonkle]

@Mario I actually ran into this myself and had to combine my two private registries into one to work with Cloudron.

[Lanhild]

@Lonkle How'd you do this?

I'm running into issues as I have multiple Cloudron servers with custom apps and only the ability to configure a single docker registry