From a dot-path notation to a YAML structure.

path_to_yaml.yaml

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

#@ def as_dict(parts, leaf):
#@   if len(parts) == 0:
#@     return leaf
#@   end
#@   return {parts[0]: as_dict(parts[1:], leaf)}
#@ end

#@ def split_dot_path(path):
#@   # "replace any dot that does NOT have a leading slash with ':::'
#@   path = regexp.replace("([^\\\])\.", path, "$1:::")
#@   path = path.replace("\.", ".")   # consume escaping of '.'
#@   return path.split(":::")
#@ end

#! original config: want to preserve all this...
---
config:
  my_app:
    other_stuff: 123
    more_stuff: true

#! merge in `image` at the location specified in `path`.
#@ image = "demo.image.url/example/test:1.0.0"
#@ path = "my_app.awesome.custom\.image.field"
#@ custom_config = as_dict(split_dot_path(path), image)

#@overlay/match by=overlay.subset({"config":{}})
---
#@overlay/match-child-defaults missing_ok=True
config: #@ custom_config