Docker: for python based images, use the -slim version

[javorszky]

After a conversation with @MichaelMcAleer and our docker initiatives, he suggested we use the -slim variant python images as base when creating the unit-python dockerfiles.

Main reason is the slim image has a lot fewer packages and therefore a lot smaller vulnerability surface and count.

The functionality would not change.

He also suggested we take a look at how ingress controller generates images: https://github.com/nginxinc/kubernetes-ingress/blob/main/.github/workflows/build-base-images.yml