Bug description

It seems there's a race condition in learn-ocaml build that prevents us from safely using the --jobs option.

To Reproduce

Steps to reproduce the behavior:

• Clone learn-ocaml (git clone https://github.com/ocaml-sf/learn-ocaml.git)
• Run cd learn-ocaml
• Docker-pull learn-ocaml master (docker pull ocamlsf/learn-ocaml:master)
• Run docker run --rm -it -v "$PWD/demo-repository:/repository" ocamlsf/learn-ocaml:master -j 2

Then, there is high probability that we get:

Learnocaml v.0.12 running.
Updating app at ./www
Cannot process exercises: Unix.Unix_error(Unix.EEXIST, "mkdir", "./www/static")

if the repository contains 2+ exercises (which is now the case of demo-repository).

Expected behavior

No error should happen.