fix!: Propagate Docker auths to BuildKit

[nderjung]

• refactor!: Migrate host Docker auth config into KraftKit's auth config
• feat(initrd): Propagate KraftKit's auths to BuildKit

Prerequisite checklist

• [ ] Read the contribution guidelines regarding submitting new changes to the project;
• [ ] Tested your changes against relevant architectures and platforms;
• [ ] Ran make fmt on your commit series before opening this PR;
• [ ] Updated relevant documentation.

Description of changes