GCloud build YAML substitutions don't work - gcloud

Question

I have a build config file that looks like:
steps:
...
<i use the ${_IMAGE} variable around 4 times here>
...
images: ['${_IMAGE}']
options:
dynamic_substitutions: true
substitutions:
_IMAGE: http://example.com/image-${_ENVIRON}
And I trigger the build like:
gcloud builds submit . --config=config.yaml --substitutions=_ENVIRON=prod
What I expected is for the gcloud to substitute the _ENVIRON variable in my script and then substitute the _IMAGE variable so that it'd expand to 'http://example.com/image-prod' - but instead I'm getting the following error:
ERROR: (gcloud.builds.submit) INVALID_ARGUMENT: generic::invalid_argument: key "_ENVIRON" in the substitution data is not matched in the template
What can I do to make that work? I really want to be able to change the environment easily with a sub and without the need to change anything in code

Answer 1

As you've seen, this isn't possible.
If the only use of _ENVIRON is by _IMAGE, why not drop the substitions from config.yaml and use _IMAGE as the substitution:
ENVIRON="prod"
IMAGE: http://example.com/image-${ENVIRON}
gcloud builds submit . \
--config=config.yaml \
--substitutions=_IMAGE=${IMAGE}