[make reset] Also remove fsroot directory

[jleveque]

- What I did

Eliminate multiple warning: failed to remove fsroot/var/lib/dpkg/info/<filename> Permission denied warning messages when calling make reset. These messages were caused because the directory is created inside the slave Docker container by superuser, so a standard user calling make reset doesn't have proper permissions to delete the directory.

- How I did it

Now we first enter the slave Docker container and remove the fsroot directory before preforming other cleanup of the repo outside the slave Docker.

- How to verify it

Run make reset and:

1. Ensure there are no warning: failed to remove fsroot/var/lib/dpkg/info/<filename> Permission denied warning messages
2. After the command completes, ensure the fsroot directory was removed