Define enviroment variables for built image

[anuraaga]

Currently there does not seem to be an option for setting ENV type of variables in the image, though OCI builders do provide the functionality and we see it in e.g., jib. The workaround is to use crane to mutate a built image.

This is the same as #800 but as it was closed, I thought it would be good to have an open issue to track the feature as I guess it would be useful.

[github-actions]

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