Document dynamic environment variables definition using Shell interpretation

[AnouckColson]

Where on docs.platform.sh should be changed?

Somewhere in https://docs.platform.sh/development/variables.html and possibly other places in the docs TBD

What exactly should be updated?

Document environment variables definition in YAML, that can dynamically refer to existing environment variables, but that can also use dynamic processing, e.g. using pipelines. See context.

Additional context

No response

[chadwcarlson]

According to the updated context link (product board), this feature is slated for Q2. Let me know if I've read this incorrectly, but otherwise moving to new milestone for Q2 rather than GA/Q1

[gilzow]

It was decided that for the initial release, we'll document that you can now reference other existing environment variables, and simple string concatenation using kubernetes syntax (e.g. $(FOO)). More dynamic expressions (pipeline, parameter expansion, string substitution, etc) are not supported. We need to mention that since the variables are defined in the app config file, the same variable precedence order is in effect