contrib: Simplify microk8s prepull YAML
contrib: Simplify microk8s prepull YAML

This YAML depended upon docker being the runtime for microk8s, which is
no longer the case in newer (>=v1.14) versions. Adjust it to just pull
via the standard mechanism rather than invoking docker.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,cilium-team/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium