app-admin/flannel: expose docker path as variable

This allows users to override the path to a local mirror.

sdk_container/src/third_party/coreos-overlay/app-admin/flannel/files/flanneld.service

@@ -12,6 +12,7 @@ RestartSec=5
 Environment="TMPDIR=/var/tmp/"
 Environment="DOCKER_HOST=unix:///var/run/early-docker.sock"
 Environment="FLANNEL_VER={{flannel_ver}}"
+Environment="FLANNEL_IMG=quay.io/coreos/flannel"
 Environment="ETCD_SSL_DIR=/etc/ssl/etcd"
 Environment="FLANNEL_ENV_FILE=/run/flannel/options.env"
 LimitNOFILE=40000
@@ -30,9 +31,9 @@ ExecStart=/usr/libexec/sdnotify-proxy /run/flannel/sd.sock \
   --env-file=${FLANNEL_ENV_FILE} \
   --volume=/usr/share/ca-certificates:/etc/ssl/certs:ro \
   --volume=${ETCD_SSL_DIR}:${ETCD_SSL_DIR}:ro \
-  quay.io/coreos/flannel:${FLANNEL_VER} /opt/bin/flanneld --ip-masq=true
+  ${FLANNEL_IMG}:${FLANNEL_VER} /opt/bin/flanneld --ip-masq=true
 
 # Update docker options
-ExecStartPost=/usr/bin/docker run --net=host --rm -v /run:/run \
-  quay.io/coreos/flannel:${FLANNEL_VER} \
+ExecStartPost=/usr/bin/docker run --net=host --rm --volume=/run:/run \
+  ${FLANNEL_IMG}:${FLANNEL_VER} \
   /opt/bin/mk-docker-opts.sh -d /run/flannel_docker_opts.env -i