Dockerfile - set ENV to result of command

Question

Is it possible to set a docker ENV variable to the result of a command? Like:

ENV MY_VAR whoami

i want MY_VAR to get the value \"root\" or

Answer 1

At this time, a command result can be used with RUN export, but cannot be assigned to an ENV variable.

Known issue: https://github.com/docker/docker/issues/29110