use pipenv instead of pip in docker build

[zkoppert]

Docker builds are giving a warning that pip should not be used as root. It suggests configuring pipenv.

WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv

see here for an example of this error as seen in the log.