backend: fix permissions and add hclfmt

After updating build-tools, I got new errors:

> ./tools/lint: line 134: hclfmt: command not found

> go install net: open /usr/local/go/pkg/linux_amd64_race/net.a:
> permission denied

This patch works around that by doing the same thing as:
https://github.com/weaveworks/build-tools/pull/44/files

backend/Dockerfile

@@ -10,8 +10,10 @@ RUN go get -tags netgo \
 		github.com/golang/lint/golint \
 		github.com/kisielk/errcheck \
 		gopkg.in/mvdan/sh.v1/cmd/shfmt \
+		github.com/fatih/hclfmt \
 		github.com/mjibson/esc \
 		github.com/client9/misspell/cmd/misspell && \
+	chmod a+wr --recursive /usr/local/go/pkg && \
 	rm -rf /go/pkg/ /go/src/
 COPY build.sh /
 ENTRYPOINT ["/build.sh"]