refactor(docker): use node slim images with major version (20|22)

After reading https://snyk.io/blog/choosing-the-best-node-js-docker-image/ , I propose this change, which follows these thoughts:

• Unify codebase across all projects to use slim images instead of alpine or normal ones. Reasonings: Security (normal images contain build tools unneeded in serving containers which introduce security issues), unification (using one type of slim|alpine simplifies code, and since alpine might cause performance and compatibility issues, I decided to go for slim)
• Instead of specifying patch or minor image versions, specify only the major version. Reasonings: Security (with only a major version specified we benefit from automatic minor and patch updates, which IMO far outweighs the potential troubles from not updating a minor or patch version due to forgetting it)
• Move from node:18 to node:20. Reasoning: node:18 has been in maintenance mode for ~1 year. node:20 will soon enter maintenance mode, but I reckoned it's too early/dangerous to push node:22 to everywhere.

This is a proposal in form of an MR. Feel free to reject it. If you do, please notify me about this, so I get the opportunity to learn from your reasoning.