You can run
reaction init to start a new local project in development, then add your custom plugin(s) to
/imports/plugins/custom. When you run
reaction locally in development or do a
docker build ... your plugin will get loaded and built just like the internal plugins do.
You can check out the example plugin to see the standard structure.
Essentially, you just need to add one (or all) of the following files and the plugin loader will import them automatically...
Also, see the accompanying docs for creating plugins/themes.