Leave the docker image .tar files in the weave directory

Making things into /var/tmp is a bit odd, and this works better in
the context of the containerized build.

Makefile

@@ -14,9 +14,9 @@ WEAVETOOLS_EXES=tools/bin
 WEAVER_IMAGE=$(DOCKERHUB_USER)/weave
 WEAVEDNS_IMAGE=$(DOCKERHUB_USER)/weavedns
 WEAVETOOLS_IMAGE=$(DOCKERHUB_USER)/weavetools
-WEAVER_EXPORT=/var/tmp/weave.tar
-WEAVEDNS_EXPORT=/var/tmp/weavedns.tar
-WEAVETOOLS_EXPORT=/var/tmp/weavetools.tar
+WEAVER_EXPORT=weave.tar
+WEAVEDNS_EXPORT=weavedns.tar
+WEAVETOOLS_EXPORT=weavetools.tar
 
 all: $(WEAVER_EXPORT) $(WEAVEDNS_EXPORT) $(WEAVETOOLS_EXPORT)