Traefik image config for version pinning, upgrades, and custom images

Accounts for the 2.9.10 security release and allows testing Traefik 3 betas.

* Use `image` to configure a specific Traefik Docker image.
* Default to `traefik:v2.9` to track future 2.9.x minor releases rather
  than tightly pinning to `v2.9.9`.
* Support images from the configured registry.

References #165

lib/mrsk/cli/traefik.rb

@@ -2,7 +2,10 @@ class Mrsk::Cli::Traefik < Mrsk::Cli::Base
   desc "boot", "Boot Traefik on servers"
   def boot
     with_lock do
-      on(MRSK.traefik_hosts) { execute *MRSK.traefik.run, raise_on_non_zero_exit: false }
+      on(MRSK.traefik_hosts) do
+        execute *MRSK.registry.login
+        execute *MRSK.traefik.run, raise_on_non_zero_exit: false
+      end
     end
   end