Simplify the prompt. Most people are going to use the Fedora package.
Hence, the prompt doesn't need to reflect the fedora-toolbox Git tree
as the current directory.
The base fedora-toolbox images for Fedoras 28, 29 and 30 are now
available from registry.fedoraproject.org. Hence it's no longer
mandatory to build them locally. If they are built locally, then the
local images will be preferred over the ones from the registry.
https://pagure.io/releng/issue/7874
It would be good to have Dockerfiles for multiple Fedora versions, and
possibly even other operating systems like Red Hat Enterprise Linux or
CentOS, so that people can use the toolbox on different host OSes.
In the future, it would be nice to have multiple toolbox containers
corresponding to different OSes exist side by side. One could then
have a Fedora 29 environment on a Fedora 28 host and so on.
