route-proxy: declare the build context its own Dockerfile has always needed

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.

modules/route-proxy/module.json

@@ -123,7 +123,11 @@
       {
         "name": "server",
         "kind": "image",
-        "from": "Dockerfile"
+        "from": "Dockerfile",
+        "context": {
+          "repository": "https://git.novox.be/novox/mesh-controller.git",
+          "ref": "main"
+        }
       },
       {
         "name": "trust",