You can use one of the templates from the templates folder. For example if you would like to run a stand alone server just use the standalone.yml file.
AFAIK when not in the container, containers is a subdirectory of /var/discourse; there is no /containers directory. So unless I’m mistaken, the line “or edit /containers/app.yml manually” in INSTALL.md should read “or edit containers/app.yml manually” or “or edit /var/discourse/containers/app.yml manually”.