Support Specifying Usage of an Existing Venv

[AgentK9]

How would this feature be useful?

Enabling Nox's session class to utilize an existing virtual environment (venv) instead of creating a new one each session would decrease setup times significantly. Distributing a pre-generated venv with projects, Docker images, or virtual machines offers notable time savings for short-lived jobs and enhances scalability in a cost-effective manner.

Describe the solution you'd like

A practical solution would be to introduce a new Nox command that either creates an empty venv or selects a pre-existing one at a given location. Enhancements to consider include:
a) Enabling the pre-installation of specific packages.
b) Allowing Nox to identify the target venv through an environment variable, command-line option, or argument.

Describe alternatives you've considered

While it's possible to bypass nox's backend by manually specifying pip installations in the noxfile, this approach leads to less clean configurations and potential conflicts with system-installed packages. An alternative method involves directing nox to use a different directory for venvs, yet this does not address the need for a singular, pre-configured venv.

I have to modify my noxfile to explicitly use session.run('pip', 'install', ... instead of session.install, which makes for messier noxfiles.

Anything else?

This request is in the context of using nox (install through pipx) is being executed inside Docker containers running on a GitLab CI/CD platform.

[cjolowicz]

Have you considered using the -r or -R flags to reuse the prepopulated environments?