Use this form of apk, since the shortcut masks errors

`apk-install` is a script that runs apk then tidies up. The problem is
that it doesn't exit with an error if the install fails, which means
that if some packages are unavailable at the time of building, it will
still all go ahead.

tools/Dockerfile

@@ -1,6 +1,7 @@
 FROM gliderlabs/alpine
 MAINTAINER Weaveworks Inc <help@weave.works>
 WORKDIR /home/weave
-RUN ["apk-install", "ethtool", "conntrack-tools", "curl", "iptables", "iproute2", "docker"]
+RUN ["apk", "add", "--update", "ethtool", "conntrack-tools", "curl", "iptables", "iproute2", "docker"]
+RUN ["sh", "-c", "rm -rf /var/cache/apk/*"]
 ADD ./weave /home/weave/
 ENTRYPOINT ["/home/weave/weave"]