ci: add artifactory container registry to test against

The artifactory container registry is configured to accept schema 2
manifests.

Signed-off-by: Justin Chadwell <me@jedevc.com>

.github/workflows/e2e.yml

@@ -98,6 +98,13 @@ jobs:
             username_secret: QUAY_USERNAME
             password_secret: QUAY_TOKEN
             type: remote
+          -
+            name: Artifactory
+            registry: buildkitghactiontests.jfrog.io
+            slug: buildkitghactiontests.jfrog.io/ghactiontest/test-docker-action
+            username_secret: ARTIFACTORY_USERNAME
+            password_secret: ARTIFACTORY_TOKEN
+            type: remote
           -
             name: Harbor
             id: harbor