diff --git a/.gitignore b/.gitignore
index 6c9406ebc5..609b440fb3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -39,9 +39,10 @@ prog/netcheck/netcheck
 tools/bin
 tools/build
 releases
+.weaver.uptodate
+.weavedns.uptodate
+.weaveexec.uptodate
 weave.tar
-weavedns.tar
-weaveexec.tar
 prog/weaveexec/weave
 prog/weaveexec/weaveproxy
 prog/weaveexec/weavewait
diff --git a/Makefile b/Makefile
index bfd649ee1e..5031dede15 100644
--- a/Makefile
+++ b/Makefile
@@ -18,23 +18,25 @@ NETCHECK_EXE=prog/netcheck/netcheck
 
 EXES=$(WEAVER_EXE) $(WEAVEDNS_EXE) $(SIGPROXY_EXE) $(WEAVEPROXY_EXE) $(WEAVEWAIT_EXE) $(NETCHECK_EXE)
 
+WEAVER_UPTODATE=.weaver.uptodate
+WEAVEDNS_UPTODATE=.weavedns.uptodate
+WEAVEEXEC_UPTODATE=.weaveexec.uptodate
+
+IMAGES_UPTODATE=$(WEAVER_UPTODATE) $(WEAVEDNS_UPTODATE) $(WEAVEEXEC_UPTODATE)
+
 WEAVER_IMAGE=$(DOCKERHUB_USER)/weave
 WEAVEDNS_IMAGE=$(DOCKERHUB_USER)/weavedns
 WEAVEEXEC_IMAGE=$(DOCKERHUB_USER)/weaveexec
 
 IMAGES=$(WEAVER_IMAGE) $(WEAVEDNS_IMAGE) $(WEAVEEXEC_IMAGE)
 
-WEAVER_EXPORT=weave.tar
-WEAVEDNS_EXPORT=weavedns.tar
-WEAVEEXEC_EXPORT=weaveexec.tar
-
-EXPORTS=$(WEAVER_EXPORT) $(WEAVEDNS_EXPORT) $(WEAVEEXEC_EXPORT)
+WEAVE_EXPORT=weave.tar
 
 WEAVEEXEC_DOCKER_VERSION=1.3.1
 DOCKER_DISTRIB=prog/weaveexec/docker-$(WEAVEEXEC_DOCKER_VERSION).tgz
 DOCKER_DISTRIB_URL=https://get.docker.com/builds/Linux/x86_64/docker-$(WEAVEEXEC_DOCKER_VERSION).tgz
 
-all: $(EXPORTS)
+all: $(WEAVE_EXPORT)
 
 travis: $(EXES)
 
@@ -66,15 +68,15 @@ $(SIGPROXY_EXE): prog/sigproxy/main.go
 $(WEAVEWAIT_EXE): prog/weavewait/main.go
 	go build -o $@ ./$(@D)
 
-$(WEAVER_EXPORT): prog/weaver/Dockerfile $(WEAVER_EXE)
+$(WEAVER_UPTODATE): prog/weaver/Dockerfile $(WEAVER_EXE)
 	$(SUDO) docker build -t $(WEAVER_IMAGE) prog/weaver
-	$(SUDO) docker save $(WEAVER_IMAGE):latest > $@
+	touch $@
 
-$(WEAVEDNS_EXPORT): prog/weavedns/Dockerfile $(WEAVEDNS_EXE)
+$(WEAVEDNS_UPTODATE): prog/weavedns/Dockerfile $(WEAVEDNS_EXE)
 	$(SUDO) docker build -t $(WEAVEDNS_IMAGE) prog/weavedns
-	$(SUDO) docker save $(WEAVEDNS_IMAGE):latest > $@
+	touch $@
 
-$(WEAVEEXEC_EXPORT): prog/weaveexec/Dockerfile $(DOCKER_DISTRIB) weave $(SIGPROXY_EXE) $(WEAVEPROXY_EXE) $(WEAVEWAIT_EXE) $(NETCHECK_EXE)
+$(WEAVEEXEC_UPTODATE): prog/weaveexec/Dockerfile $(DOCKER_DISTRIB) weave $(SIGPROXY_EXE) $(WEAVEPROXY_EXE) $(WEAVEWAIT_EXE) $(NETCHECK_EXE)
 	cp weave prog/weaveexec/weave
 	cp $(SIGPROXY_EXE) prog/weaveexec/sigproxy
 	cp $(WEAVEPROXY_EXE) prog/weaveexec/weaveproxy
@@ -82,7 +84,10 @@ $(WEAVEEXEC_EXPORT): prog/weaveexec/Dockerfile $(DOCKER_DISTRIB) weave $(SIGPROX
 	cp $(NETCHECK_EXE) prog/weaveexec/netcheck
 	cp $(DOCKER_DISTRIB) prog/weaveexec/docker.tgz
 	$(SUDO) docker build -t $(WEAVEEXEC_IMAGE) prog/weaveexec
-	$(SUDO) docker save $(WEAVEEXEC_IMAGE):latest > $@
+	touch $@
+
+$(WEAVE_EXPORT): $(IMAGES_UPTODATE)
+	$(SUDO) docker save $(addsuffix :latest,$(IMAGES)) > $@
 
 $(DOCKER_DISTRIB):
 	curl -o $(DOCKER_DISTRIB) $(DOCKER_DISTRIB_URL)
@@ -112,7 +117,7 @@ publish: $(PUBLISH)
 
 clean:
 	-$(SUDO) docker rmi $(IMAGES)
-	rm -f $(EXES) $(EXPORTS)
+	rm -f $(EXES) $(IMAGES_UPTODATE) $(WEAVE_EXPORT)
 	rm -f test/tls/*.pem
 
 build:
diff --git a/build/Dockerfile b/build/Dockerfile
index 562673c855..23861bdfcd 100644
--- a/build/Dockerfile
+++ b/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.
diff --git a/site/building.md b/site/building.md
index 04b998ab49..4b91d59f79 100644
--- a/site/building.md
+++ b/site/building.md
@@ -57,8 +57,7 @@ $ make
 
 This will build the weave components and package them into three
 Docker images (`weaveworks/weave`, `weaveworks/weavedns`, and
-`weaveworks/weaveexec`).  These are then exported (as `weave.tar`,
-`weavedns.tar` and `weaveexec.tar`).
+`weaveworks/weaveexec`).  These are then exported as `weave.tar`.
 
 ## Building in a Docker container
 
@@ -171,15 +170,13 @@ you can do so with
 $ vagrant ssh -c 'make -C src/github.com/weaveworks/weave'
 ```
 
-you should then find container snapshot tarballs in the top-level
-directory. You can use these snapshots with `docker load` against a
-different host, e.g.
+you should then find a `weave.tar` container snapshot tarball in the
+top-level directory. You can use that snapshot with `docker load`
+against a different host, e.g.
 
 ```bash
 $ export DOCKER_HOST=tcp://<HOST:PORT>
 $ docker load < weave.tar
-$ docker load < weavedns.tar
-$ docker load < weaveexec.tar
 ```
 
 You can provide extra Vagrant configuration by putting a file
diff --git a/test/setup.sh b/test/setup.sh
index d2b49f0175..7235b4a485 100755
--- a/test/setup.sh
+++ b/test/setup.sh
@@ -10,8 +10,6 @@ echo "Copying weave images, scripts, and certificates to hosts, and"
 echo "  prefetch test images"
 for HOST in $HOSTS; do
     docker_on $HOST load -i ../weave.tar
-    docker_on $HOST load -i ../weavedns.tar
-    docker_on $HOST load -i ../weaveexec.tar
     run_on $HOST mkdir -p bin
     cat ../bin/docker-ns | run_on $HOST sh -c "cat > $DOCKER_NS"
     cat ../weave | run_on $HOST sh -c "cat > ./weave"