Correct issues with build of Docker images for DMOD services and Python sources

[robertbartel]

At present, Docker images for the py-sources stack do not build successfully. These build and assemble dependencies for the DMOD service Docker images in the main stack (e.g., that of the scheduler service). The result is that new service images also cannot be built.

Initially, there are at least two things causing trouble (though there could be others not yet fully apparent):

• recent changes in later versions of pip and in how Python package are built (as discussed in this ngen issue)
• usage of a rather old base image: python:3.8-alpine3.15

Note also that it is anticipated that there will be trouble with service images related to the image builds not completely accounting for version restrictions of certain dependencies (e.g. pandas) as required by some of the DMOD internal packages (see #400), though strictly speaking, we haven't had that trouble yet.

[robertbartel]

PR #420 introduces a fix that it was hoped would resolve the image build problems this issue tracks. It did not, though the changes are still worth doing and could still be a necessary part of the entire eventual solution.

[robertbartel]

Closed by #427.