Return a 502 when container is down

If the app container is down or not responding then traefik will return
a 404 response code. This is not ideal as it suggests a client rather
than a server problem.

To fix this, we'll define a catch all route that always returns a 502.

This is not ideal as this route would take priority over a shorter route
with priorty 1.

TODO: up the priority of the app route.

test/integration/app_test.rb

@@ -10,8 +10,7 @@ class AppTest < IntegrationTest
 
     kamal :app, :stop
 
-    # traefik is up and returns 404s when it can't match a route
-    assert_app_not_found
+    assert_app_is_down
 
     kamal :app, :start
 
@@ -51,7 +50,6 @@ class AppTest < IntegrationTest
 
     kamal :app, :remove
 
-    # traefik is up and returns 404s when it can't match a route
-    assert_app_not_found
+    assert_app_is_down
   end
 end

test/integration/integration_test.rb

@@ -55,12 +55,6 @@ class IntegrationTest < ActiveSupport::TestCase
       assert_app_version(version, response) if version
     end
 
-    def assert_app_not_found
-      response = app_response
-      debug_response_code(response, "404")
-      assert_equal "404", response.code
-    end
-
     def wait_for_app_to_be_up(timeout: 20, up_count: 3)
       timeout_at = Time.now + timeout
       up_times = 0