The Dockerfile's own comment already said it — 'the build context is the
mesh-controller repository root' — but nothing in the manifest actually
said so to the mesh, so every build attempt used mesh-catalog's own
directory instead and failed with 'stat go.mod: file does not exist'.
Never caught before because route-proxy has never been assigned anywhere.
Uses the context mechanism just added (mesh-controller#62), proven
working tonight on builder's own self-build.