Enhance st2ctl Docker detection and usage hints (#4988)

[Emu55]

Enhance st2ctl Docker detection and usage hints

This pull request adds support for detecting when st2ctl is being run inside a Docker container and, for VM-style commands, immediately exits with a clear error message and guidance on the proper Docker commands to use.

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Background

The st2ctl script was originally written to manage StackStorm services on a single host or VM, using tools like ps, systemctl, or direct process checks. In containerized deployments, each StackStorm component runs in its own Docker container, so those VM-style operations:

• Fail or return misleading results when run inside a container
• Don’t reflect the recommended Docker-native workflow

Issue #4988 requests that st2ctl detect this scenario and provide helpful Docker-specific guidance instead of attempting unsupported operations.

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What’s Changed

1. Docker detection helper
   Added a new function running_in_docker() which returns success if either:

   • The file /.dockerenv is present, or
   • The string “docker” appears in /proc/1/cgroup

2. Usage suggestion helper
   Added suggest_docker_usage(subcommand) which:

   • Prints a red-highlighted error:

     Error: “st2ctl <subcommand>” is not supported inside Docker.
     

   • Prints examples of the correct Docker commands to manage services:

     docker ps
     docker-compose restart st2api
     docker-compose logs -f st2stream
     

   • Exits with status code 1

3. Early exit for VM-style commands
   Inserted a pre-dispatch check before the main case "$1" in … esac block:

   if running_in_docker; then
     case "$1" in
       status|start|stop|restart|restart-component|reopen-log-files|reload)
         suggest_docker_usage "$1"
         ;;
     esac
   fi

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