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  • Christopher Werner thinks this is interesting:

-e "MY_VAR=docker-was-here"

From

Cover of Docker in Production: Lessons from the Trenches

Note

Looks like a typo here - I don't think that the -e option is meant to be used in this example. The next example illustrates how -e puts the environment variable into the container, whereas this one illustrates that the environment variable is otherwise unknown in the container