test: use proper shell variable in minikube script
test: use proper shell variable in minikube script

Users may prefer to use docker alternatives, by setting DOCKERCMD
variable, which is set by 'common.sh'. Use it in 'copy_image_to_cluster'
instead of plain 'docker'.

Tested with DOCKERCMD=podman (podman-2.1.1).

Signed-off-by: Shachar Sharon <b4f835f8fb3f8aa558cc3d7fb3e13b91034b92f2@redhat.com>

rook/rook,rook/nfs,rook/nfs,rook/nfs,rook/rook,rook/rook,rook/rook,rook/rook