github.com/docker/docker-ce@v17.12.1-ce-rc2+incompatible/components/cli/scripts/warn-outside-container (about)

     1  #!/usr/bin/env bash
     2  set -eu
     3  
     4  target="${1:-}"
     5  
     6  if [[ -z "${DISABLE_WARN_OUTSIDE_CONTAINER:-}" ]]; then
     7      (
     8          echo
     9          echo
    10          echo "WARNING: you are not in a container."
    11          echo "Use \"make -f docker.Makefile $target\" or set"
    12          echo "DISABLE_WARN_OUTSIDE_CONTAINER=1 to disable this warning."
    13          echo
    14          echo "Press Ctrl+C now to abort."
    15          echo
    16      ) >&2
    17      sleep 10
    18  fi