Running debugger instance for ubuntu 20 does not set up the iRODS repo

[JustinKyleJames]

When I run the debugger instance for ubuntu 20 (build_debuggers.ubuntu20.Dockerfile) it does not set up the iRODS repo. The packages can't be installed from the local directories because it can't install the external packages.

I had to set up this up manually by doing the following:

1. Install gnupg
2. Set up the iRODS repo.

At that point I could install the packages.

[SwooshyCueb]

This is by design. The build_debuggers.* Dockerfiles produce images that are meant to be used as alternate base images for building the irods_runner.* Dockerfiles. Doing so will result in the debugger images.

It appears the README needs to be updated, however.