FIX: `/var/lib/docker` doesn't exist on macOS (#543)

FIX: /var/lib/docker doesn’t exist on macOS (#543)

Even though docker info --format '{{.DockerRootDir}}' returns that path.

diff --git a/launcher b/launcher
index 91a39b9..8aaed23 100755
--- a/launcher
+++ b/launcher
@@ -235,8 +235,7 @@ check_prereqs() {
   # 7. enough space for the bootstrap on docker folder
   folder=`$docker_path info --format '{{.DockerRootDir}}'`
   safe_folder=${folder:-/var/lib/docker}
-  test=$(($(stat -f --format="%a*%S" $safe_folder)/1024**3 < 5))
-  if [[ $test -ne 0 ]] ; then
+  if [[ -d $safe_folder && $(stat -f --format="%a*%S" $safe_folder)/1024**3 -lt 5 ]] ; then
     echo "You have less than 5GB of free space on the disk where $safe_folder is located. You will need more space to continue"
     df -h $safe_folder
     echo

GitHub sha: 138f0dff0d6cc7149bfd852fbba33a6366eae7df

This commit appears in #543 which was approved by SamSaffron. It was merged by SamSaffron.