Improve CI buildtimes

[duncdrum]

CI build times for an automated release is above 35min. The time necessary for generating the ToC assets (~3min), doesn't allow for much optimisation and depends entirely on how complex of an update is required by a given commit. The checkout with 10min is suspect, but fixed for now. However, 20min are spend testing via docker. We could reduce this by publishing a frus image and utilise a published cache to speed this up considerably.