[WIP] Try to speed up the CI for testing by building and caching in ghcr

[suricactus]

No description provided.

[duke-nyuki]

Task linked: QF-4434 Try to optimize CI test builds

[anajoy]

Build with ghcr.io caching of qfieldcloud images did run (test.yml):
7 m 07 s (app: 1 m 31 s, nginx: 19 s, qgis: 5 m 17 s)
2 m 49 s (app: 2 m 15 s, nginx: 21 s, qgis: 0 m 13 s)
3 m 30 s (app: 2 m 36 s, nginx: 41 s, qgis: 0 m 13 s)
3 m 15 s (app: 2 m 23 s, nginx: 36 s, qgis: 0 m 16 s)
6 m 39 s (app: 1 m 21 s, nginx: 21 s, qgis: 5 m 57 s, reaching 10 GB of cached layers
It looks like, I have reached the limit of available github action caches of 10 GB. Deleting all caches for pr 972 to retest caching
8 m 59 s (app: 2 m 26 s, nginx: 24 s, qgis: 7 m 09 s, first ghcr.io cache created , size 2.22 GB
8 m 53 s (app: 2 m 31 s, nginx: 19 s, qgis: 6 m 03 s, cache not used ?, size 4.44 GB
2 m 54 s (app: 2 m 29 s, nginx: 18 s, qgis: 0 m 17 s, cache used !, size 4.44 GB

Build without ghcr.io caching of qfieldcloud images did run (test_baseline.yml):
6 m 40 s (app + nginx + qgis build_and_push without the push step)
6 m 10 s (app + nginx + qgis build_and_push without the push step)
6 m 10 s (app + nginx + qgis build_and_push without the push step)
6 m 07 s (app + nginx + qgis build_and_push without the push step)
7 m 23 s (app + nginx + qgis build_and_push without the push step)
6 m 45 s (app + nginx + qgis build_and_push without the push step)