Admin process should be seen as a one-off process (opposed to long running processes that make up an application).
Usually used for maintenance task, though a REPL, admin process must be executed on the same release (codebase + configuration) than the application.
In the docker-compose file we could define an admin service that is ran at the same time as the application and in which we could jump (docker exec -ti ADMIN_CONTAINER_ID bash) to execute some admin tasks. The container is able to access all the other containers of the application (provided it belongs to the same networks)