Consistently use Docker creds for fetching images

ci/tasks/generate-changelog.yml

@@ -4,11 +4,9 @@ image_resource:
   type: registry-image
   source:
     repository: springio/github-changelog-generator
-    tag: '0.0.7'
-    registry_mirror:
-      host: ((registry-mirror-host))
-      username: ((registry-mirror-username))
-      password: ((registry-mirror-password))
+    tag: '0.0.8'
+    username: ((docker-hub-username))
+    password: ((docker-hub-password))
 inputs:
   - name: git-repo
   - name: artifactory-repo

ci/tasks/promote-version.yml

@@ -5,6 +5,8 @@ image_resource:
   source:
     repository: springio/concourse-release-scripts
     tag: '0.3.4'
+    username: ((docker-hub-username))
+    password: ((docker-hub-password))
 inputs:
   - name: git-repo
   - name: artifactory-repo