postgres builds its own runtime, like any other module

Its provisioner container named an image nobody could produce — a zero digest
placeholder. It names an artifact instead, and the module says how to build it,
so the mesh can make the database provider the catalogue needs.

modules/postgres/module.json

@@ -102,7 +102,6 @@
       "id": "runtime",
       "type": "container",
       "name": "mesh-postgres",
-      "image": "mesh-runtime-postgres@sha256:0000000000000000000000000000000000000000000000000000000000000000",
       "network": "postgres",
       "volumes": [
         "/var/lib/mesh/postgres/broker:/run/secrets/broker:ro",
@@ -114,7 +113,21 @@
         "MESH_PROVISION_PASSWORD_FILE": "/run/secrets/superuser",
         "MESH_BROKER_FILE": "/run/secrets/broker",
         "MESH_RECEIVES": "/var/lib/postgres/grants/mesh.json"
-      }
+      },
+      "artifact": "runtime",
+      "args": [
+        "run",
+        "/app/modules/postgres/dist/provisioner/index.js"
+      ]
     }
-  ]
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "runtime",
+        "kind": "image",
+        "from": "Dockerfile"
+      }
+    ]
+  }
 }