`make-disk-image` is a tool for creating VM images. It takes an argument
`contents` that allows one to specify files and directories that should
be copied into the VM image. However, directories end up not at the
specified target, but instead at a subdirectory of the target, with a
nix-store-like path, e.g.
`/target/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-source`. See issue
https://github.com/NixOS/nixpkgs/issues/226203 .
This change adds a test for make-disk-image's contents directory
handling and adds a fix (appending `/` to rsync input directory names).
This closes issue https://github.com/NixOS/nixpkgs/issues/226203 .
