For images running on Kubernetes, there is no guarantee on how duplicate
environment variables in the image config will be handled. This seems
to be different from Docker, where the last environment variable value
is consistently selected.
The current code for `streamLayeredImage` was exploiting that assumption
to easily propagate environment variables from the base image, leaving
duplicates unchecked. It should rather resolve these duplicates to
ensure consistent behavior on Docker and Kubernetes.
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