Make the project build using the committed lock dir

[Leonidas-from-XIV]

Since there is a lock.dir in the project and Dune can build the project with package management, this changes the Dockerfile to use dune pkg for building it.

Currently it is experiencing some kind of issue:

0.213 File "dune.lock/lock.dune", line 1, characters 0-0:                                                                                                                                                                                                                       
0.213 Error: The lock dir is not sync with your dune-project
0.213 Hint: run dune pkg lock

I do not experience the issue outside the Docker container and it works when using autolocking so there seems to be some kind of issue with the lock directory.