dockerTools: Fix layer redundancy.

When building an image with multiple layers, files
already included in an underlying layer are supposed to
be excluded from the current layer. However, some subtleties
in the way filepaths are compared seem to be blocking this.

Specifically:
* tar generates relative filepaths with directories ending in '/'
* find generates absolute filepaths with no trailing slashes on directories

That is, paths extracted from the underlying tarball look like:
    nix/store/.../foobar/
whereas the layer being generated uses paths like:
    /nix/store/.../foobar

This patch modifies the output of "tar -t" to match the latter format.