Previously, proxy-deps and go-deps included the source tree for local
projects. This can cause build conflicts when files are renamed.
By adopting a multi-stage build for the proxy-deps image, we can be sure
that we only preserve essential dependencies & manifests in the
proxy-deps and go-deps images.
Furthermore, `bin/update-go-deps-shas` and `bin/update-proxy-deps-shas` have
been added to ease maintenance when files are changed.
Fixes#159
Signed-off-by: Oliver Gould <ver@buoyant.io>