WIP Add caching for private Tangle docker images

[eike-hass]

Description of change

Add a caching layer for the docker images we pull for setting up the private tangle in every job. We expect those to change very unfrequently from run to run and it looks like we safe about ~70% runtime for ~150MB of cache.

Type of change

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

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [X] Enhancement (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Fix

How the change has been tested

Tested in CI.

Change checklist

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

• [X] I have followed the contribution guidelines for this project
• [X] 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
• [X] New and existing unit tests pass locally with my changes