Support using build config (username, password, ssh key).

[cdrage]

Is your enhancement related to a problem? Please describe

We have build config on the bootc-image-builder: https://github.com/osbuild/bootc-image-builder?tab=readme-ov-file#-build-config but not in GUI.

Describe the solution you'd like

We should have this, so we no longer have to do echo root:root | chpasswd in the Containerfile for logging in.

Discussions need to happen whether we want to support it being a "form passed" typing-in-form, or passing in a config file..

Describe alternatives you've considered

No response

Additional context

No response