ci: optimise macOS run times

[myitcv]

Currently, our use of Docker dominates CI runs. This is true for macOS too.

The rate limiting step on macOS however is that by default Docker for Desktop uses half the available number of CPU cores and a fraction of the memory.

To remove these limits we need to be able to alter the configuration of Docker for Desktop in a headless way.

https://github.com/docker/for-mac/issues/2359#issuecomment-702055818

[tmc]

https://github.com/tmc/d4mctl might help

[myitcv]

Thanks @tmc - your project certainly fills in the missing blank with respect to how/where this is configured.