Fix pipenv detection by updating dockerfile to install pipenv

[btalanski]

By adding pip3 install --user pipenv to Dockerfile, the UI is now able to detect pipenv version properly.

I noticed this after seeing the following error msg in the logs:

helper <ERROR> pipenv binary not found at: /usr/local/bin/pipenv

[MrBrax]

good idea, but unfortunately the server itself doesn't use pipenv and i never figured out (or really spent much time figuring out) how to do it, so it would only really bloat the image until it's fully implemented

[btalanski]

Should it be removed or maybe commented out for now ?

[MrBrax]

yeah i just haven't had the energy to work on this for a while, and the pipenv thing has been neglected since release

[btalanski]

I'll update the PR later.