Optimize size and rebuild behavior of OCI image

[amotl]

Optimize image size

Each command creates a layer in the docker image that contains the changes to the previous layer.
All files are still left in the previous layer and space is saved in the image.
To fix this, run apt install, pip install, and apt remove in a single RUN command so that files are not kept in hidden layers.

Originally posted by @intermittentnrg in #24 (comment)

Optimize rebuild behavior

Currently, pip install is copying all python files from /src to elsewhere so it now needs to run and will install dependencies when any .py/application file is changed. Changes in Python should not re-run pip etc., only if setup.py (or README.rst) were changed.

Originally posted by @intermittentnrg in #24 (comment) and #24 (comment)

[amotl]

Dear @intermittentnrg,

thank you for your attention to detail. I am currently travelling, but I will be happy to address those topics on the next iteration in one way or another.

With kind regards,
Andreas.