What is the reason for expect not succeeding when the docker container is executed without using the --interactive flag?

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The expect command is used to automate interactive tasks. When the docker container is executed without the --interactive flag, it runs in non-interactive mode, which means that it does not allocate a TTY (teletype) for the container, and the standard input is closed. Therefore, expect cannot interact with the container as there is no interactive session available. Without a TTY, the input sent to the docker command is not considered as interactive, so expect fails to send any input to the container.