As image names have a pattern in 15-SP2 (SLE*), change the script that release to TEST to use this image name pattern instead of list all images.This will avoid frequently updates in script cause by new image package submissions.
How to validate the yaml
For some reason, valid yaml is not supported - the indentation required is rather wild. But to check the syntax before commit, you can use the plugin locally:
wget https://github.com/tomzo/gocd-yaml-config-plugin/releases/download/0.9.0/yaml-config-plugin-0.9.0.jar
java -jar yaml-config-plugin-0.9.0.jar syntax sp1-stagings.gocd.yaml | grep -q '"valid":true'