Cache docker images for CI

We loose quite a lot of time waiting for the image downloads, i.e. alone the quay.io/jetstack/cert-manager-acmesolver:v0.11.0 image takes up to 2,5 minutes !

  Normal  Pulling    8m21s  kubelet, ci-399-helmreleases-test  Pulling image "quay.io/jetstack/cert-manager-acmesolver:v0.11.0"
  Normal  Pulled     5m45s  kubelet, ci-399-helmreleases-test  Successfully pulled image "quay.io/jetstack/cert-manager-acmesolver:v0.11.0"

Another example gathered (from #609 (closed)):

I looked at current master installation and saw that pulling the rocketchat image alone (439MB) took 8 minutes !

root@master:~# kubectl -n oas-apps describe pod rocketchat-rocketchat-857c9d5c9f-7xfwz
  Normal   Pulling    41m                kubelet, master    Pulling image "open.greenhost.net:4567/openappstack/openappstack/rocketchat:3.2.2-096aa0023"
  Normal   Pulled     33m                kubelet, master    Successfully pulled image "open.greenhost.net:4567/openappstack/openappstack/rocketchat:3.2.2-096aa0023"

root@master:~# crictl images |grep rocket
open.greenhost.net:4567/openappstack/openappstack/rocketchat                3.2.2-096aa0023         70286d69ba81b       439MB

We could create a local caching registry:

https://mayeu.me/post/saving-time-bandwidth-caching-docker-images-with-local-registry/