#### Docker env. Synonyms for variables in env file?

Let suppose I have an env file with

DB_PW=123


And a docker command that expects

-env MYSQL_ROOT_PASSWORD=123


Is there a way to take that DB_PW from the env file and assign it to MYSL_ROOT_PASSWORD in the command line? (obviously not having to add a line to the env file with the explicit assignment)
I have been reading here and there, but I am not sure if it’s possible or where this would be documented.

Source: Docker Questions