Set PIP_USER=true in the image

[Daniel Hollas]

When using pip to install packages, we always user the --user flag so that the packages are installed in ~/.local/ and thus saved in a docker volume that backs the home directory.

We should just make it the default, so that in case user runs pip directly for whatever reason, they automatically install to ~/.local/ even if they forget to specify --user. (This happened to me personally many times)

See:
https://pip.pypa.io/en/stable/topics/configuration/#environment-variables