Flynn is natively highly available and multi-host, which makes doing volume storage tricky. Our current immediate plan for this is to support NFS mounts for apps so that you can point at an external file server (or service like AWS EFS). Feel free to watch or thumbs-up this issue: https://github.com/flynn/flynn/issues/1521
As far as plugins go, Flynn is entirely API-driven so you should be able to hook in and do whatever you want without modifying Flynn itself. If you run into something that you want to do that makes sense as an integration but isn't currently possible, please open an issue and we can discuss how to make it happen.
> The other thing that has come in very handy is being able to deploy Dockerfile builds directly, is this something that Flynn can do as well, or is support planned?
We support pushing Docker images to a registry built-in to Flynn: https://flynn.io/docs/docker
Building Dockerfiles is currently tricky from a security and modularity perspective as it requires running inside a privileged Docker daemon. There is no way that I'm aware of to do a build without providing root or root-like capabilities. If it was as simple as importing a package and using it in an unprivileged container to perform the build, we'd definitely evaluate adding support carefully.