Add docs about overriding host with host file

doc/set-up-gdk.md

@@ -94,6 +94,10 @@ You can override the port used by this GDK with a 'port' file.
     echo 4000 > port
+Similarly, you can override the host (for example if you plan to use GDK inside a Docker container).
+
+    echo 0.0.0.0 > host
+
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