Append to a string as yaml

config.yml

#@ load("@ytt:data", "data")
---
#! Input
apiVersion: v1
kind: Secret
metadata:
  name: secret
stringData:
  longYaml: |
    ---
    first: [a, b, c]
    second:
      foo: bar
      ree: baz
    third: true
      

overlay.yml

#@ load("@ytt:overlay", "overlay")
#@ load("@ytt:yaml", "yaml")

#@ def applyToYamlString(left, right):
#@   leftYaml = yaml.decode(left)
#@   new = overlay.apply(leftYaml, right)
#@   return yaml.encode(new)
#@ end

#@overlay/match by=overlay.subset({"kind": "Secret", "metadata": {"name": "secret"}})
---
stringData:
  #@overlay/replace via=applyToYamlString
  longYaml:
    #@overlay/match missing_ok=True
    fourth:
      blah: dee
      doo: bee