release build broken on Docker 1.6

By default, tmpfs on Docker 1.6 is 64mb which is too
small for Go builds on the Kube project (binary size, etc).

This moves the release build to use a non tmpfs work dir.

build/build-image/Dockerfile

@@ -22,8 +22,12 @@ ENV GOARM 5
 ENV GOOS    linux
 ENV GOARCH  amd64
 
+# work around 64MB tmpfs size in Docker 1.6
+ENV TMPDIR /tmp.k8s
+
 # Get the code coverage tool and godep
-RUN go get golang.org/x/tools/cmd/cover github.com/tools/godep
+RUN mkdir $TMPDIR && \
+    go get golang.org/x/tools/cmd/cover github.com/tools/godep
 
 # We use rsync to copy some binaries around.  It is faster (0.3s vs. 1.1s) on my
 # machine vs. `install`