make CNI setup work on recent Docker-for-Macs

Docker for Mac 1.12.0-rc3 (as of beta18) looks for mounted paths on VM
first, and when it doesn't find them there, it assumes they refer to
paths on the Mac OS host. At which point it will complain that the
paths don't exit.

So instead we mount / since that will always exist on the VM. It's
also what we used to do already when launching the router.

Fixes #2434.

weave

@@ -135,20 +135,19 @@ docker_run_options() {
     echo --privileged --net=host $(docker_sock_options)
 }
 
-cni_volume_options() {
-    [ "$1" = "setup" -o "$1" = "setup-cni" ] && echo -v /etc/cni:/etc/cni -v /opt/cni:/opt/cni
-}
-
-launch_router_options() {
-    [ "$1" = "launch" -o "$1" = "launch-router" ] && echo -v /:/host -e HOST_ROOT=/host
+exec_options() {
+    case "$1" in
+        setup|setup-cni|launch|launch-router)
+            echo -v /:/host -e HOST_ROOT=/host
+            ;;
+    esac
 }
 
 exec_remote() {
     docker $DOCKER_CLIENT_ARGS run --rm \
         $(docker_run_options) \
         --pid=host \
-        $(cni_volume_options "$@") \
-        $(launch_router_options "$@") \
+        $(exec_options "$@") \
         -e DOCKERHUB_USER="$DOCKERHUB_USER" \
         -e WEAVE_VERSION \
         -e WEAVE_DEBUG \
@@ -1034,7 +1033,7 @@ create_cni_script() {
 #!/bin/sh
 docker run --rm $(docker_run_options) --pid=host -i \\
  -e CNI_VERSION -e CNI_COMMAND -e CNI_CONTAINERID -e CNI_NETNS \\
- -e CNI_IFNAME -e CNI_ARGS -e CNI_PATH $(cni_volume_options setup) \\
+ -e CNI_IFNAME -e CNI_ARGS -e CNI_PATH -v /etc/cni:/etc/cni -v /opt/cni:/opt/cni \\
  $PLUGIN_IMAGE $COVERAGE_ARGS $2
 EOF
     chmod a+x "$1"
@@ -1050,12 +1049,12 @@ EOF
 }
 
 setup_cni() {
-    if [ -d /opt/cni/bin ] ; then
-        create_cni_script /opt/cni/bin/weave-net  --cni-net
-        create_cni_script /opt/cni/bin/weave-ipam --cni-ipam
+    if [ -d $HOST_ROOT/opt/cni/bin ] ; then
+        create_cni_script $HOST_ROOT/opt/cni/bin/weave-net  --cni-net
+        create_cni_script $HOST_ROOT/opt/cni/bin/weave-ipam --cni-ipam
     fi
-    if [ -d /etc/cni/net.d -a ! -f /etc/cni/net.d/10-weave.conf ] ; then
-        create_cni_config /etc/cni/net.d/10-weave.conf
+    if [ -d $HOST_ROOT/etc/cni/net.d -a ! -f $HOST_ROOT/etc/cni/net.d/10-weave.conf ] ; then
+        create_cni_config $HOST_ROOT/etc/cni/net.d/10-weave.conf
     fi
 }