Use EL: #{request.contextPath}
.
It's quite useful for creating navigation links, to set in your main template a Facelets variable :)
<ui:param name="root" value="#{request.contextPath}/" />
UPDATE: It's not recommended to use the full path available in the app server because it's not guaranteed to be the same URL the user is using to access your app, so, beware of that.
If you really want, though, you can do that, using some methods available in HttpServletRequest to create an String like this:
#{request.scheme}://#{request.serverName}:#{request.serverPort}#{request.contextPath}