Mounting files to be used during features build

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