That worked great!
(for posterity: I needed to tweak the quoting a bit as well, like so:
./launcher rebuild app --docker-args "-e DISCOURSE_SMTP_PASSWORD=$DISCOURSE_SMTP_PASSWORD"
I got **ERROR** - 535 Authentication failed otherwise - and in the last line of the rebuild console output you can see there’s an extra set of quotes in the command line. I think the env_file variable declaration probably should also not have quotes around it, unlike my original post.)
thanks muchly for the help and have a wonderful day!