Dockerfile: Fix python installation process on latest alpine

[weswigham]

The Dockerfile build has been failing for like a month and a half, and I honestly just didn't bother to look into why until today, since there weren't any balance changes to speak of (that I cared about). Turns out, the way the Dockerfile was linking up python just doesn't work anymore on latest alpine. So now it just uses the system py3-pip package to install pip and a python virtual environment for the bloodytools dependencies themselves.

Maybe related: Should there just be a task in the nightly CD workflow that just runs a docker build . to catch integration issues like this?

[Bloodmallet]

Thank you very much!