The YTT experiments

input.yml

version: '3.7'

---
#@ load( "@ytt:overlay", "overlay" )
#@overlay/match by=overlay.all
#@overlay/match-child-defaults missing_ok=True
---

#@overlay/assert
version: '3.7'

services:
  cool-container:
    labels:
      - "cold"
      - "ice"
      - "snow"

---
#@ load( "@ytt:overlay", "overlay" )
#@overlay/match by=overlay.all
#@overlay/match-child-defaults missing_ok=True
---
  
#@overlay/assert
version: '3.7'

services:
  hot-container:
    labels:
      - "warm"
      - "lava"
      - "fire"

output

# cat ./first.yaml | ytt --file -
version: "3.7"
services:
  cool-container:
    labels:
    - cold
    - ice
    - snow
  hot-container:
    labels:
    - warm
    - lava
    - fire