Prune Docker image

[HebaruSan]

The kspckan/netkan image inherits a lot of cruft from python:3.11: autoconf, automake, make, dpkg-dev, gcc, g++, hicolor-icon-theme, imagemagick, subversion, tcl/tk, lots of libs, x11-common, etc., as well as the package download cache in /home/netkan/.cache/pip. This bloats it quite a bit:

REPOSITORY                         TAG       IMAGE ID       CREATED              SIZE
kspckan/netkan                     latest    4883cd42823b   3 days ago           1.11GB

Now it starts from ubuntu:latest, installs only the Python components it needs for each step, and excludes the pip cache from the copy-forward of /home/netkan, resulting in a much slimmed down image (~31% of previous):

REPOSITORY                         TAG       IMAGE ID       CREATED              SIZE
infra_testing                      latest    13fa9e747ffa   About a minute ago   355MB

This may slightly speed up redeploying the bot.

Note that this will downgrade Python from 3.11 to 3.10, which will not matter in the short term, but over the long term it will automatically get future Python versions as they are added to ubuntu:latest and the image is rebuilt.

[techman83]

Nothing stands out, looks great! Thanks @HebaruSan