How to 'docker plugin install' for json-based plugin
config.json - this is a config, that describes .sock format configs, and not the .json format (please correct me if I'm wrong)
I've verified it working with .spec files, not very sure how it works with json files though. For .spec files, you don't mention .spec files in config.json. That is used only for unix socket plugins (option 1). Infact there is no need to have config.json for TCP socket plugins.
how do I create the rootfs and why do I need it if my plugin is just a standalone REST service and it is not even in the container?
In my understanding, rootfs is only for unix socket plugins. Plugin discovery works out of the box if .spec files exists in the right folder. In nutshell, you just create spec file and put it in the right discovery folder and try to bootstrap the container with that plugin name. You don't have to run commands like "docker plugin create/install/enable". You run the server, put the file in right folder and let new containers use that plugin.