bump docker version in weave-build docker image

to 1.3.1, the same version we use everywhere else, e.g. in weavexec.

build/Dockerfile

@@ -1,6 +1,9 @@
 FROM ubuntu
 
-RUN apt-get -y update && apt-get -y install --no-install-recommends build-essential git ca-certificates docker.io mercurial libpcap-dev curl make pkg-config gcc bison flex
+RUN apt-get -y update && apt-get -y install --no-install-recommends ca-certificates apt-transport-https
+RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 36A1D7869245C8950F966E92D8576A8BA88D21E9
+RUN echo deb https://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list
+RUN apt-get -y update && apt-get -y install --no-install-recommends build-essential git lxc-docker-1.3.1 mercurial libpcap-dev curl make pkg-config gcc bison flex
 
 # When doing a build in a container, "apt-get update" happens twice,
 # which can be a very significant overhead for incremental builds.