RCON is enabled by default, so you can exec into the container to access the Minecraft server console:

docker exec -i mc rcon-cli

Note: The -i is required for interactive use of rcon-cli.

To run a simple, one-shot command, such as stopping a Minecraft server, pass the command as arguments to rcon-cli, such as:

docker exec mc rcon-cli stop

The -i is not needed in this case.

If rcon is disabled you can send commands by passing them as arguments to the packaged mc-send-to-console script. For example, a player can be op'ed in the container mc with:

docker exec mc mc-send-to-console op player
            |                     |
            +- container name     +- Minecraft commands start here

In order to attach and interact with the Minecraft server, add -it when starting the container, such as

docker run -d -it -p 25565:25565 --name mc itzg/minecraft-server

With that you can attach and interact at any time using

docker attach mc

and then Control-p Control-q to detach.

For remote access, configure your Docker daemon to use a tcp socket (such as -H tcp://0.0.0.0:2375) and attach from another machine:

docker -H $HOST:2375 attach mc

Unless you're on a home/private LAN, you should enable TLS access.