Add note about monorepo benchmark to hacking doc

[gridbugs]

No description provided.

[Alizter]

I think some more details are needed about which docker commands to run after building the image.

I got as far as to build the image and run:

$ docker run name_of_image

then I go to bench/monorepo however the first issue is that dune is required, not the bootstrapped one:

$ make bench
dune build _build/default/bench.exe --release
make: dune: No such file or directory
make: *** [Makefile:6: _build/default/bench.exe] Error 127

putting some dune into the environment allows me to continue but now:

$ make bench
dune build _build/default/bench.exe --release
_build/default/bench.exe /home/user/dune/_build/default/bin/main.exe build -j auto
Fatal error: exception Unix.Unix_error(Unix.ENOENT, "create_process", "/home/user/dune/_build/default/bin/main.exe")
make: *** [Makefile:9: bench] Error 2

Not sure how to proceed form here.

Also the docker image is not named after being built and I had to explicitly docker tag it.

[gridbugs]

I fixed an issue with the benchmarks where if a _build directory was present inside bench/monorepo make bench would fail. This was the problem @Alizter ran into above.

I've also added the full docker commands I use to build and run the image.