Change docker to registry. It's more general

Signed-off-by: Taylor Silva <tsilva@pivotal.io>

pipelines/build-and-push-simple-image.yml

@@ -16,8 +16,8 @@ resources:
     # You need to provide these variables when
     # setting the pipeline
     repository: ((image-repo-name))/simple-image
-    username: ((docker-username))
-    password: ((docker-password))
+    username: ((registry-username))
+    password: ((registry-password))
 
 jobs:
 - name: build-and-push