Use docker hint, not requirement

steps/find_skymodel_cal.cwl

@@ -46,6 +46,8 @@ outputs:
         outputEval: $(JSON.parse(self[0].contents))
 
 requirements:
-  - class: DockerRequirement
-    dockerPull: 'prefactor:latest'
   - class: InlineJavascriptRequirement
+
+hints:
+  DockerRequirement:
+    dockerPull: lofar/lofar-pipeline