Ability to control build parallelism

[arvola]

It would be very useful to have a way to control how much Forge parallelizes builds. At minimum viable, Forge should allow turning parallelization off.

Background:
Running builds in parallel can lead to the following Docker bug: moby/moby#9656 - depending on the number of builds and if they share file system layers. It can make builds take an order of magnitude longer. In my case, a 5 minute build could take up to 2 hours.

[rhs]

This should be fixed in 0.4.14. You can set the concurrency parameter in forge.yaml (it now defaults to 5 so you may not need to set it).