So, after some struggling, I managed to make all the stuff working.
I must premise: although I’m a “old generation” developer (first '80), I always looked for the best and new form to develop or manage, so I really hate in 2021 to write weird commands full of cryptic options like with the old cp/m - dos, I always look for some interface that make my life easier and clearer.
Then, I use e.g. Portainer to manage containers, it permits to start/stop/edit/duplicate all containers on the fly, without “bashing” up and down the filesystem in search of one-in-a-million file; e.g. to change the container network it’s just to choose one from a list and make a click, idem for adding some parameters, or a volume like in the example @tophee wrote; for this reason I tried NPM, because I prefer to “contain” my Nginx proxy and because apparently with just a few click, well understanding what I’m doing but without to remember a new set of weird command-and-options.
Back to my Discourse container, I had to “discourse-setup” it again; evertything went right, “evil” Postgres installed in version 13, no “drunk unicorn” (I’m sorry but thinking about an “unicorn” running in my server makes me laugh! ), in short all went the right way. Then, I made the modification in order to make Discourse running with websocket: everything ok also this time. Fortunately, previous Discourse setup made automatic backups, so with just few clicks I restored everything (the most I use Discourse, the most I love it!).
I had to try several times the setting for NPM, in the beginning I had some problem with certificates, but in the end even it run well.
I added a second proxy pointing to my Wordpress container (yes, I’m “containing” everything, I like the idea of a most clean server with all the main packages contained in a managed place), and even it run well.
So, in the end, I have my server (a VPS) with its email server (I also tried to “contain” it, but after weeks of harsh fight, I gave up), Discourse pointing to it, Worpress running in another container, and NPM managing both: all on my server, without depending on other (and much, much more expensive) services for deploing, emailing, etc.
Next step, importing some hundred thousand post from the “old good Phpbb”: prepare yourself for other posts from me!
A great thank to @tophee and @pfaffman for the help, I can understand how difficult can it be to help people when they work in non standard way like me.