Python environment related failure with Docker image builds using recent version of ngen

[robertbartel]

As discussed here, there are some issues with images and recent ngen versions. This probably stems from NOAA-OWP/ngen#755,. Others are experiencing trouble also: see CIROH-UA/NGIAB-HPCInfra#12 and CIROH-UA/NGIAB-CloudInfra#137.

PR #629 is likely to introduce a (at least temporary) fix by making the ngen builds and eventually the entrypoint use a virtual environment, though one without any changes from the default environment. There are many reasons why this is not ideal. We should probably try to get back to the default environment. If it becomes clear that this isn't really an option, we should at minimum improve how the ngen and other images set up the virtual environment, as well as document in various places (inline and elsewhere) that this is how things function.