I think that the only solution is to build an image that includes the plugins that you want, push it to a docker repo, launch it with the appropriate ENV variables, and at some point see that the database is migrated and assets precompiled and pushed to S3 on (at least) the first time the image is run. You might want to migrate once with SKIP_POST_DEPLOYMENT_MIGRATIONS set while the old image is running and then again after the new image has launched and the old ones have shut down with SKIP_POST_DEPLOYMENT_MIGRATIONS turned off or run the db:ensure_post_migrations rake task.
You can run a rake task in one of your running images, something like
rake api_key:create_master['description of key']
The above might be enough to get you a bit further. I’ve run kubernetes instances for clients on GCP and AWS in the past. I was never 100% happy with how it worked (it worked perfectly from the client’s perspective, it just wasn’t very elegant from mine, but it wasn’t so inelegant that I bothered to fix it, either!). I don’t have much more to offer here, but feel free to contact me directly if you need more help.