Improve preflight file permissions check (#114)

* Added startup permissions check with more helpful message

* Updated README

* Fixes faulty test

rel/overlays/bin/check_file_permissions

@@ -0,0 +1,3 @@
+#!/bin/sh
+cd -P -- "$(dirname -- "$0")"
+exec ./pinchflat eval Pinchflat.Release.check_file_permissions

rel/overlays/bin/docker_start

@@ -1,4 +1,11 @@
 #!/bin/sh
+/app/bin/check_file_permissions
+
+if [ $? -ne 0 ]; then
+  echo "Filesystem error. Exiting."
+  exit 1
+fi
+
 /app/bin/migrate
 
 cd -P -- "$(dirname -- "$0")"