This is an Alpine based rcon container, using mcrcon as client.
Run it specifying host, port and password along with a command (or -t for a interactive terminal):
docker run elamperti/docker-rcon -H <server_ip> -P <port> -p <password> "say Hello world" save-all
The client is able to take the server address, port and password from environment variables MCRCON_HOST, MCRCON_PORT and MCRCON_PASS.