makefile: use a variable to choose "docker" command
makefile: use a variable to choose "docker" command

This change allows the use of alternatives to or wrappers around
the normal docker command for container builds.

Example 1: make image-rbdplugin CONTAINER_CMD=podman
Example 2: CONTAINER_CMD=podman make image-rbdplugin

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi