Use docker credentials for fetching changelog image

This commit updates the CI pipeline to use the configured docker
credentials for fetching the changelog-generator CI image for the
relevant task.

ci/tasks/generate-changelog.yml

@@ -4,7 +4,9 @@ image_resource:
   type: registry-image
   source:
     repository: springio/github-changelog-generator
-    tag: '0.0.7'
+    tag: '0.0.8'
+    username: ((docker-hub-username))
+    password: ((docker-hub-password))
 inputs:
 - name: git-repo
 - name: artifactory-repo