brew.sh: allow Docker to run as root.

Library/Homebrew/brew.sh

@@ -350,6 +350,9 @@ fi
 check-run-command-as-root() {
   [[ "$(id -u)" = 0 ]] || return
 
+  # Allow Docker to do everything as root (as it's normal there)
+  [[ -f /proc/1/cgroup ]] && grep docker -q /proc/1/cgroup && return
+
   # Homebrew Services may need `sudo` for system-wide daemons.
   [[ "$HOMEBREW_COMMAND" = "services" ]] && return