You actually don’t need another /var/[myforum]… You can just use the same /var/discourse for multiple instances.
/var/discourse/containers/app.yml → This is the default instance you get by running discourse-setup
/var/discourse/containers/myforum.yml → This is the 2nd instance you get by running cp app.yml myforum.yml && edit && ./launcher bootstrap myforum
When editing myforum.yml, make sure you put a different host path under the volumes section, so that your 2nd container stores its data under a different directory than the 1st one.