don't attach rewrite_etc_hosts helper container to weave

...since that is a) inefficent and b) can block when IPAM hasn't had
consensus.

We can only end up in this situation when DOCKER_HOST is pointing at
the weave proxy and we are running `weave run|attach` with
`--local`. So it's very much an edge case.

Fixes #1858.

weave

@@ -1169,7 +1169,7 @@ rewrite_etc_hosts() {
     CONTENTS="$(etc_hosts_contents $FQDN "$@")"
     # rewrite /etc/hosts, unlinking the file (so Docker does not modify it again) but
     # leaving it with valid contents...
-    docker run --rm --net=none --privileged \
+    docker run --rm --net=none -e WEAVE_CIDR=none --privileged \
         -v $CONTAINERS_PATH:$MNT \
         --entrypoint=sh \
         $EXEC_IMAGE -c "echo '$CONTENTS' > $MNT_HOSTS && rm -f $MNT_HOSTS && echo '$CONTENTS' > $MNT_HOSTS"