Configure docker proxy URL for K3s-based installation to overcome docker pull rate limit

Change-Id: I94dde54e4f124369daaf40ff6863b77e2fd25d23
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/installers/full_install_osm.sh b/installers/full_install_osm.sh
index 1b18334..6cde383 100755
--- a/installers/full_install_osm.sh
+++ b/installers/full_install_osm.sh
@@ -429,6 +429,7 @@
         FATAL_TRACK k8scluster "install_cluster_addons.sh failed for kubeadm cluster"
     elif [ "${K8S_CLUSTER_ENGINE}" == "k3s" ]; then
         K3S_INSTALL_OPTS="-i ${OSM_DEFAULT_IP} -D ${OSM_DEVOPS} ${DEBUG_INSTALL}"
+        [ -n "${DOCKER_PROXY_URL}" ] && K3S_INSTALL_OPTS="${K3S_INSTALL_OPTS} -p ${DOCKER_PROXY_URL}"
         # The K3s installation script will automatically take the HTTP_PROXY, HTTPS_PROXY and NO_PROXY,
         # as well as the CONTAINERD_HTTP_PROXY, CONTAINERD_HTTPS_PROXY and CONTAINERD_NO_PROXY variables
         # from the shell, if they are present, and write them to the environment file of k3s systemd service,