don't embed docker binary

We needed it because some of our invocations of Weave Net's `weave`
script depended on it, but that is no longer the case as of Weave Net
2.1.

Fixes #2974.

docker/Dockerfile.cloud-agent

@@ -4,7 +4,6 @@ LABEL works.weave.role=system
 WORKDIR /home/weave
 RUN apk add --update bash conntrack-tools iproute2 util-linux curl && \
 	rm -rf /var/cache/apk/*
-ADD ./docker /usr/local/bin/
 ADD ./weave ./weaveutil /usr/bin/
 COPY ./scope /home/weave/
 ENTRYPOINT ["/home/weave/scope", "--mode=probe", "--no-app", "--probe.docker=true"]