if I'm maintaining my plugin as a separate project from reaction, is there a way to install that plugin into a docker container at deploy time? I'm trying to avoid cloning all of reaction locally and creating a custom build image manually.
No, there’s no avoiding creating a custom build. Deploy time and build time are 2 separate steps. You can’t/shouldn't do an additional install step at deploy time. However, there’s no reason you can’t automate that custom build.