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.

lib/kamal/commands/traefik.rb

@@ -6,6 +6,14 @@ class Kamal::Commands::Traefik < Kamal::Commands::Base
   DEFAULT_ARGS = {
     'log.level' => 'DEBUG'
   }
+  DEFAULT_LABELS = {
+    # These ensure we serve a 502 rather than a 404 if no containers are available
+    "traefik.http.routers.catchall.entryPoints" => "http",
+    "traefik.http.routers.catchall.rule" => "PathPrefix(`/`)",
+    "traefik.http.routers.catchall.service" => "unavailable",
+    "traefik.http.routers.catchall.priority" => 1,
+    "traefik.http.services.unavailable.loadbalancer.server.port" => "0"
+  }
 
   def run
     docker :run, "--name traefik",
@@ -97,7 +105,7 @@ class Kamal::Commands::Traefik < Kamal::Commands::Base
     end
 
     def labels
-      config.traefik["labels"] || []
+      DEFAULT_LABELS.merge(config.traefik["labels"] || {})
     end
 
     def image