Remove old compiled spt3g/so3g layers from Docker image

[BrianJKoopman]

Description

This PR rebases the ocs Docker image onto the ubuntu:22.04 base image. It also sets up and enables a Python virualenv for the ocs installation.

Motivation and Context

This has been long overdue, but historically we built upon the simonsobs/so3g image, which is built upon the simonsobs/spt3g image, which in turn was built upon ubuntu:22.04.

This chain of images was to facilitate installation of the spt3g/so3g stack, which prior to so3g becoming pip installable needed to be compiled. Each layer compiled its respective package. Now that we can (and do) install so3g from pip, and so3g brings its own spt3g with it, we don't need these base layers.

There were a few issues with this historical stack still being in place, the largest of which is that somehow in the build process Python 3.10 was being replaced with Python 3.8. (3.10 is present in simonsobs/so3g, but only 3.8 is present in simonsobs/ocs.) This may have exacerbated the setuptools issue in #391.

This also saves a bunch of disk space, since we don't have everything needed to compile spt3g/so3g in the image anymore. Compare these two builds to see it's about 1.2GB!

REPOSITORY        TAG                  IMAGE ID       CREATED              SIZE
ocs               main                 6885fb6b5fe3   About a minute ago   2.67GB
ocs               rework               96e71b04582c   About a minute ago   1.46GB

The virtualenv setup also prepares us for the update to ubuntu:24.04, which comes with Python 3.12 which enforces uses of virtual environments.

How Has This Been Tested?

Builds and tests have been run locally and all pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.