podman/docs/source/markdown/options/env-merge.md

####> This option file is used in: ####> podman create, run ####> If file is edited, make sure the changes ####> are applicable to all of those.

--env-merge=env

Preprocess default environment variables for the containers. For example if image contains environment variable hello=world user can preprocess it using --env-merge hello=${hello}-some so new value is hello=world-some.

Please note that if the environment variable hello is not present in the image, then it'll be replaced by an empty string and so using --env-merge hello=${hello}-some would result in the new value of hello=-some, notice the leading - delimiter.