ci: allow manual Docker builds

[daniel-mader]

Description of change

Temporarily allow manually triggered Docker builds. Useful for testing versions for unmerged branches.

Links to any relevant issues

n/a

How the change has been tested

n/a

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for 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
• 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
• I have successfully tested this change in a docker environment