Mounting files to be used during features build

[myslak71]

I'd like to be able to mount some files which then be later used during feature installation.

My use case looks as follows:

1. Before running devcontainer up I download some files
2. Then custom feature needs to do some processing on downloaded files - but it fails since those files are not available during build

I can see that within the BuildKit there is an option that we can leverage:
https://docs.docker.com/reference/dockerfile/#run---mount

[chrmarti]

Depending on the kind / size of artifacts the feature could do the downloading or include them in its package.