Remove <include path="**" /> from appengine-web.xml.
When this line is present in appengine-web.xml, CapeDwarf will treat _everything_ as a static file (no servlet will ever be invoked). I'm trying to figure out if that's correct or not. Here's what GAE docs say:
Any request for a URL whose path matches a static file serves the file directly to the browser—even if the path also matches a servlet or filter mapping.
Appspot doesn't follow this rule, apparently.