Don't update image if local repository changes

The dockerfile has
* Remote apt repositories.
* Local apt repositories

If the remote apt repositories change then the docker image doesn't have to be rebuilt (however it can be rebuilt with --no-cache).
If the local apt repository changes then the docker image must be rebuilt (because it uses COPY).

This is problematic because:
* Some users have a workflow where they have --local-repository and --destination as the same directory to allow them to build multiple repositories that depend on each other. This is problematic because if you build repo1 successfully it deposits the .debs into it's local repository and invalidates it's cache.
* The user may want to just run "apt update && apt upgrade" for faster updating rather than rebuilding the image.

The local repository stage must happen at build time before other dependencies are installed because a dependency in the control file might only exist in the local repository.