add support for build-arg files

[deitch]

- What I did

Added support for specifying build args to a package build from another file as --build-arg-file file1. You can specify multiple files as --build-arg-file file1 --build-arg-file file2 ....

This allows interesting patterns, like a build.yaml that is usable for multiple structures and versions of something (kernel builds are on my mind) while changing some build args.

linuxkit pkg build --build-yml build.yaml --build-arg-file build-args-1 ./pkg/foo
linuxkit pkg build --build-yml build.yaml --build-arg-file build-args-2 ./pkg/foo

Or if you want to get kernel-specific:

linuxkit pkg build --build-yml build-kernel.yaml --build-arg-file build-args-6.6.x ./kernel
linuxkit pkg build --build-yml build-kernel.yaml --build-arg-file build-args-5.15.x ./kernel
linuxkit pkg build --build-yml build-kernel.yaml --build-arg-file build-args-6.6.x --build-arg-file build-args-debug ./kernel

It fits very well with the linuxkit philosophy, as everything still is in files, and can be tracked via VCS.

In theory, I could have achieved the same result by allowing stacking of build.yml files, but that is messy to do. Maybe in the future.

I added tests, of course.

- How I did it

Writing code. Writing tests. Happy they pass.

- How to verify it

CI

- Description for the changelog

Support for multiple build-args files