Allow dockerfiles to exist anywhere, and there can be multiple ones

[ElectreAAS]

This PR solves #395. Users can tell us where their dockerfile is, and we'll setup a "dockerfile": "path/to/dockerfile" in production.conf and it should be good to go.

If they want to have multiple dockerfiles, it's now possible:

{
  "name": "org/repo",
  "dockerfile": "path/to/normal/dockerfile"
},
{
  "name": "org/repo",
  "dockerfile": "path/to/other/dockerfile"
}

Tested locally and everything seems to be in order.

[art-w]

Very nice, thank you! We should update the readme with the list of extraordinary settings that one can ask for when integrating with current-bench :) (custom dockerfiles, ocaml versions, single vs multicore, github labels, ...)