Include util-linux package in weaveexec container

weaveexec/Dockerfile

@@ -1,7 +1,7 @@
 FROM gliderlabs/alpine
 MAINTAINER Weaveworks Inc <help@weave.works>
 WORKDIR /home/weave
-RUN ["apk", "add", "--update", "ethtool", "conntrack-tools", "curl", "iptables", "iproute2"]
+RUN ["apk", "add", "--update", "ethtool", "conntrack-tools", "curl", "iptables", "iproute2", "util-linux"]
 RUN ["sh", "-c", "rm -rf /var/cache/apk/*"]
 ADD ./weave /home/weave/
 ADD ./sigproxy /home/weave/