[GSProcessing] Make SageMaker image build faster and smaller.

[thvasilo]

Issue #, if available:

Description of changes:

• We switch away from using pipenv in the SageMaker image, relying instead on on the poetry-generated requirements.txt file to install dependencies.
• Also switch to images having prod target by default (although this requires poetry to be installed on the image building environment).
• Update out-of-date README files.

Testing

Unit tests+Ran a SageMaker job with ml-25m data.

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[jalencato]

LGTM, the performance number matches our expectation as well.