do you know discourse.org forum software? Other libre forum software?

Simply Mastodon, Telegram, Riot. :)

Discourse is free software (GPLv2[0]), so it is acceptable on that front. Note that it requires JavaScript to be enabled, although the JavaScript is free software it may be an issue to some people. I dislike the trend of web sites requiring JavaScript when they could be made to work without.

Discourse heavily advertises their paid plans, which are hosted offerings (on a dot-org page, no less - traditionally the dot-com site is for the commercial offering and the dot-org is the community side!). You need to find the small link on their page to the GitHub to access the source code, where they have instructions on how to install[1].

Docker is also free software (Apache 2.0[2]), albeit what is known as "open core" - the free version is termed "community edition" and their main offering is "enterprise edition." Docker, like Discourse, tries to heavily push the commercial offering on you, and even when you want the "community edition" they try to direct you to "Docker Store." Fortunately, Docker is available in Trisquel 8.0 as "docker.io."

Another problem with Docker is that it offers a "hub[5]" which is sort of like a package repository. Many images (I'd wager the vast majority) offered through the "hub" are free software, but there's no guarantee that everything on there is. Fortunately, this is probably not as big a problem as it would be for a user-facing package manager, because you're unlikely to be looking through a directory of images offered on the hub; instead you already have an application in mind you'd like to install and want to find out if it's on the hub.

[0] https://github.com/discourse/discourse/blob/master/LICENSE.txt
[1] https://github.com/discourse/discourse/blob/master/docs/INSTALL.md
[2] https://www.docker.com/components-licenses
[3] https://www.docker.com/docker-software-end-user-license-agreement
[4] https://hub.docker.com/