Provide to mount files from git with stapel

[onegreyonewhite]

Before proceeding

• I didn't find a similar issue

Problem

Now it is only possible to add files to the image. It would be cool if stapel provide to use @from_source/some_dir (as example) in mount directive with link to some dir in git project.

Cases where this may be necessary:

1. Use pem file for sign or generate smthg.
2. Use binaries from lfs for some build steps.

It would be convenient to add mounting from dependent images with invalidation on change. Like fromImage but with mounting to subdir.

Solution (if you have one)

No response

Additional information

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