Ensure Docker images remain up to date

Description

Changes the slim image builder and Dockerfiles to ensure that all images and software remain up-to-date, for security purposes.

Changes

• stable-slim is now the base image, instead of buster-slim
• building images ignores the cache and pulls the latest version of stable-slim so all software are up-to-date

Test and Review

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such