Regardless of the software you choose, @BobBooth1 is right volunteering is the way to go. But the chances of needing Kubernetes for that are pretty close to 0. I've been told the Esri Kubernetes implementation is geared for large roll outs and that it's expensive. So you are very unlikely to find a little watershed group or a land trust that wants it. You'd be better off going all in on ArcGIS Online. (Not interesting to me at all.)
I'd suggest you try Docker instead of Kubernetes unless you have already mastered it. You can run Docker on a laptop or your own server. You can spend years learning it without ever touching kubernetes. I've been working on putting ArcGIS Enterprise into Docker since 2017, off and on. (Longer than Esri! 🙂 See GitHub - Wildsong/docker-arcgis-enterprise: A set of dockers for ESRI Arcgis Enterprise
It has more stars and forks on github than all my other projects combined, and it's not even done! It kind of works. It's 100% free, you "just" have to have licenses for the Esri components you want to use. I set it up to allow deploying one instance of each component, to allow easy testing and learning. I happily accept suggestions and improvements.
I asked several people at Esri for support for this project in the form of a developer account and was stonewalled. I don't have the extra pocket money to pay another year to develop this for them. I retire soon and will have no reason to work on it other than "fun" so I plan on changing direction.
That's why I suggested geoserver and its friends to you. They are actually much more fun than Esri. The developers are mostly friendly and helpful. It's just not Esri, so you won't be working in the same world. This is the tradeoff.
BTW all the open source tools already run in Docker containers... so any Docker skills you learn transfer to a very wide range of other projects.
