Consulting the kubernetes plugin readme you'll find the following statements
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Note that it was previously possible to define containerTemplate but that has been deprecated in favor of the yaml format.

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and
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Unlike scripted k8s template, declarative templates do not inherit from parent template. Since the agents declared at stage level can override a global agent, implicit inheritance was leading to confusion.

You need to explicitly declare the inheritance if necessary using the field inheritFrom.

In the following example, nested-pod will only contain the maven container.

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In the following there's an example about where there's no inheritance happening. But there's no single example about how to use inheritFrom with declarative Pipeline.
Tried to define a name for the outer POD to inherit from using several different things, but seems like I did not find the right place to do so.

Please improve the documentation making it more clear about how to use this feature (if it is supposed to work).