Set PROD_STANDY to read the production database

- 172.17.0.1 is the default gateway of Docker bridge network
- 54321 is an arbitrary port for an SSH tunnel that can be created with:

    ssh -L 172.17.0.1:54321:localhost:65400 pink -N # production slave

It depends on metabrainz/musicbrainz-server#1337

build/musicbrainz-dev/DBDefs.pm

@@ -84,6 +84,14 @@ MusicBrainz::Server::DatabaseConnectionFactory->register_databases(
         host        => "db",
         port        => "5432",
     },
+    # How to connect for read-only access to the production database.
+    PROD_STANDBY => {
+        database    => "musicbrainz_db",
+        username    => "musicbrainz_ro",
+        password    => "",
+        host        => "172.17.0.1",
+        port        => "54321",
+    },
     # How to connect for administrative access
     SYSTEM    => {
         database    => "template1",