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

test/cli/traefik_test.rb

@@ -3,7 +3,8 @@ require_relative "cli_test_case"
 class CliTraefikTest < CliTestCase
   test "boot" do
     run_command("boot").tap do |output|
-      assert_match "docker run --name traefik --detach --restart unless-stopped --publish 80:80 --volume /var/run/docker.sock:/var/run/docker.sock --log-opt max-size=\"10m\" traefik:v2.9.9 --providers.docker --log.level=DEBUG", output
+      assert_match "docker login", output
+      assert_match "docker run --name traefik --detach --restart unless-stopped --publish 80:80 --volume /var/run/docker.sock:/var/run/docker.sock --log-opt max-size=\"10m\" #{Mrsk::Commands::Traefik::DEFAULT_IMAGE} --providers.docker --log.level=DEBUG", output
     end
   end