chore(dockerfile): Follow up on #2024 implementing @polarathene's suggestions

[georglauterbach]

Description

Follow up on #2024.

@polarathene Note that I enabled auto-merging again :D You may turn it off if there is more you'd like to discuss.

Type of change

• Improvement (non-breaking change that does improve existing functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes