Let Poetry use a virtualenv in docker container

[ChristopherChudzicki]

Description/Context

When we originally set up Poetry in Open Discussions, we told Poetry not to use a virtualenv. In retrospect, I think this was a mistake:

• makes installing packages awkward (must install with -u root)
• Poetry maintainers indicate lack of virtual env causes problems Document why virtual environments should (nearly) always be used, and system site-packages is considered harmful python-poetry/poetry#6398 (comment)

Additionally:

• OCW Studio and MITxOnline both use a virtualenv with poetry, so changing this would bring us in-line with our other projects

Plan/Design

Follow approach outlined in https://github.com/mitodl/ocw-studio/blob/master/Dockerfile