Cache pip install in python Dockerfiles

[mdavis-xyz]

Fixes #460

I'm not sure if I should copy models before app.py, or app.py first. It probably doesn't matter. (Models is larger. But does it change more often?)

Note that I haven't tested these changes, because they're simple. I'm assuming CICD will tell me if I've broken stuff.

[mndeveci]

Thanks for your contribution! Can you apply same changes to other python runtimes?

[mdavis-xyz]

I thought I did apply it to all lambda runtimes and templates (which didn't have it already). I just searched the code repo for all files with "COPY requiremnets.txt app.py ./" (or something like that.)

[mndeveci]

@mdavis-xyz no worries, there is this one for python3.9 for instance https://github.com/aws/aws-sam-cli-app-templates/blob/master/python3.9/hello-img/%7B%7Bcookiecutter.project_name%7D%7D/hello_world/Dockerfile which is not included in this PR