What action does 'docker rmi' command require when a specified image is in use by a container?

When you attempt to remove an image using the docker rmi command and the specified image is currently in use by one or more containers, Docker will return an error indicating that the image cannot be removed because it is being used.

To successfully remove the image, you need to take one of the following actions:

Stop and Remove the Containers: First, stop any running containers that are using the image, and then remove those containers. You can do this with the following commands:
```
docker stop <container_id_or_name>
docker rm <container_id_or_name>
```
Force Removal: If you want to remove the image regardless of the containers using it, you can use the -f (force) flag with the docker rmi command. This will remove the image and any associated containers:
```
docker rmi -f <image_name_or_id>
```

Be cautious when using the force option, as it will delete the containers that are using the image without confirmation.