Merge pull request #5733 from MikeMcQuaid/docker-root

brew.sh: allow Docker to run as root.

Dockerfile

@@ -32,7 +32,6 @@ RUN cd /home/linuxbrew/.linuxbrew \
 	&& cd /home/linuxbrew/.linuxbrew/Homebrew \
 	&& git remote set-url origin https://github.com/Homebrew/brew
 
-USER linuxbrew
 WORKDIR /home/linuxbrew
 ENV PATH=/home/linuxbrew/.linuxbrew/bin:/home/linuxbrew/.linuxbrew/sbin:$PATH \
 	SHELL=/bin/bash \

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