Fix pipenv + pip 18.1 breakage

[ryankennedy]

The image is currently broken due to a compatibility issue between pipenv and pip 18.1 (pypa/pipenv#2924). The solution (until it's fixed within pipenv itself) is to make sure that the version of pip installed in the virtualenv is 18.0

pipenv run pip install pip==18.0

[bryan-rhm]

Having the same issue, setting pipenv run pip install pip==18.0 did not work

[ryankennedy]

Hmm, it worked for my situation. Regardless, pipenv 2018.10.9 is out which, I believe, fixes the issue. Maybe try upgrading pipenv itself now and see if that helps? That's what I'm going to try now with this Docker image.

[ryankennedy]

Unfortunately it appears pipenv v2018.10.9 may break the --site-packages command line flag (pypa/pipenv#2992).