Support custom registry better

[Schille]

What is the new feature about?

There are workarounds required to get Gefyra running in an air-gapped environment with private registry.
See gefyra install (for the operator/stowaway/carrier image)
See gefyra run (for the cargo image)

Why would such a feature be important to you?

Finding the right hooks to plug in custom artifact URLs is hardly possible.

Anything else we need to know?

nope