NodeBB offers well-known and extensible F/OSS forum software for your growing community. NodeBB is trusted by some large communities, and has extensibility built in with lots of other features for you to use.
NodeBB (code) is a very popular community forum that's easy to host and extensible. Built with NodeJS, developing for it is easy to do and it's got a permissive license (GPL3.0) that encourages experimentation.
It looks great, and is trusted by large orgs like Opera:
Along with simply looking good, NodeBB has documentation to read through along with decent intall instructions for common platforms like Ubuntu. There's also a useful demo application you can try if you want to get a feel for the product.
Along with being a great self-hostable tool, NodeBB offers professional "one click" paid hosting. It's not cheap, but it is an option for those with the budget and the community size.
NodeBB is unfortunately not so
docker friendly (there is no official documentation/quick start guide on using
docker) but there is an official container image at least. There's also a
docker-compose.yml in the repository but it's not very prescriptive.
That said NodeBB's documentation is full of examples on how to install for various platforms:
So you can either pave your own way with
docker (and piece together the configuration schemes and how to use the docker image) or use some built-in options.