Solaris there's no official way of doing it, though there are a few things possible.
The most important thing to note is that the database isn't multisite compatible. You will always need one database per forum.
Apart from that I can confirm it's possible to host multiple Flarum installs with a single instance of the codebase, but it requires a few changes. I made a modified version of Flarum that does just that to test the output of my Flarum migration service without installing multiple Flarums or switching the database of a single install for each test.
Don't ask me for the code though, the way it works is quite specific to my migration service (which is closed source) and there are plans to maybe make it into some kind of Premium Flagrow extension in the future.
It's interesting to note that every Flarum hosting service so far has been using one install per tenant (in particular: FreeFlarum). One install per tenant has the advantage of having isolated extension installs so you don't have to upgrade everybody at once. It's also probably safer with extensions that write directly on the filesystem. And of course if using containers this has many other advantages.
If you're just interested in the ability to update all forums at once and not the ability to easily create gazillions of forums, you could just symlink the composer.json, composer.lock and vendor folder of all installs, so that running Composer commands in one of them applies to all of them.
