Issue title: Document command variable for the cli container
 Issue body: There is an ability to configure which command to run upon `cli` start. It should be documented.
 Comments: 
 Comment 0: @achekulaev was this documented as apart of #543 ?
 Comment 1: No, it wasn't. It is not an environment variable, but rather a way to heavily customize compose file.
https://docs.docker.com/compose/compose-file/compose-file-v2/#command
 Comment 2: Here's an example of the approach:
https://github.com/docksal/example-nodejs/blob/master/.docksal/docksal.yml#L10
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