weave setup to download all required docker images

Closes #308.

README.md

@@ -52,7 +52,13 @@ On `$HOST1` run (as root)
     host1# C=$(weave run 10.0.1.1/24 -t -i ubuntu)
 
 The first line starts the weave router, in a container. This needs to
-be done once on each host.
+be done once on each host. The required docker image for the weave
+router container is downloaded automatically. There is also a `weave
+setup` command for downloading this and other images required for
+weave operation; this is a strictly optional step which is especially
+useful for automated installation of weave and ensures that any
+subsequent weave commands do not encounter delays due to image
+downloading.
 
 The second line runs our application container. We give it an IP
 address and network, in

weave

@@ -5,6 +5,7 @@ SCRIPT_VERSION="(unreleased version)"
 
 usage() {
     echo "Usage:"
+    echo "weave setup"
     echo "weave launch     [-password <password> | -passwordfile <path>] <peer> ..."
     echo "weave launch-dns <cidr>"
     echo "weave connect    <peer>"
@@ -447,6 +448,11 @@ DOCKER_VERSION_MINOR=$(echo "$DOCKER_VERSION" | cut -d. -f 2)
 DOCKER_VERSION_PATCH=$(echo "$DOCKER_VERSION" | cut -d. -f 3)
 
 case "$COMMAND" in
+    setup)
+        for img in $IMAGE $DNS_IMAGE $TOOLS_IMAGE ; do
+            docker pull $img
+        done
+        ;;
     launch)
         check_not_running $CONTAINER_NAME $BASE_IMAGE
         create_bridge