mmctl can be downloaded independently and installed directly on your work computer, so you do not need to install it on the server.
The latest release (brandnew!) is 7.7.0 which can be downloaded directly from the GitHub releases page:
mmctl stores local configuration in a .config folder and therefore you should not run it directly in the container.
Depending on the configuration of your Mattermost server container, the so-called “local mode” could be enabled though and if this is the case, then Mattermost does not need to store authentication information and you can use the mmctl binary from within the container directly using socket authentication (no need to specify auth tokens then).
To see if this is the case, you could run the command as follows from within your container: