Which Docker command allows you to update an already running container?

The command that allows you to update an already running container is specifically designed to execute a command within that container and can be used to modify the container’s state or behavior while it is executing. When using docker exec, you can start a new process in a running container, which means you can effectively make changes or interact with the existing application without stopping the container.

For instance, if you want to change configurations, run scripts, or install software inside a running container to reflect new requirements, docker exec is the command that facilitates this real-time adjustment. It provides the ability to interact and control the current state of the application in the container while it remains active.

Considering the other options, docker update serves a different purpose by allowing you to change the resource limits of a container, such as CPU or memory, but it does not "update" the application or service running inside the container. Similarly, docker start is used to start a stopped container, and docker modify is not a valid Docker command at all. Thus, in the context of altering operations or state within an already active container, docker exec stands out as the appropriate command.