Make get.docker.io install directions slightly better by telling user exactly how to add themselves to the "docker" group

As a bonus, if the user has run this script the way we recommend (ie, without "sudo" or "su", run as their user), we can actually give them the exact command they'll need with their correct username substituted appropriately.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

hack/install.sh

@@ -37,8 +37,10 @@ if command_exists docker || command_exists lxc-docker; then
 	( set -x; sleep 20 )
 fi
 
+user="$(id -un 2>/dev/null || true)"
+
 sh_c='sh -c'
-if [ "$(whoami 2>/dev/null || true)" != 'root' ]; then
+if [ "$user" != 'root' ]; then
 	if command_exists sudo; then
 		sh_c='sudo sh -c'
 	elif command_exists su; then
@@ -124,6 +126,16 @@ case "$lsb_dist" in
 				$sh_c 'docker run busybox echo "Docker has been successfully installed!"'
 			) || true
 		fi
+		your_user=your-user
+		[ "$user" != 'root' ] && your_user="$user"
+		echo
+		echo 'If you would like to use Docker as a non-root user, you should now consider'
+		echo 'adding your user to the "docker" group with something like:'
+		echo
+		echo '  sudo usermod -aG docker' $your_user
+		echo
+		echo 'Remember that you will have to log out and back in for this to take effect!'
+		echo
 		exit 0
 		;;