Add ability to disable concurrency

[alehaa]

Building a docker image for multiple platforms at once will execute the builds in parallel. However, for continuous integration it would be nice to execute the builds in serial to not get the build log mixed up.

I know, that serial builds are supported by just running individual jobs for each platform. However, using multiple platforms in one job allows me to get the manifest list created and pushed, too.

[tonistiigi]

#2049

[therc]

Is there some magic directive that can be used in a Dockerfile to reduce concurrency? 99% of the time I want it, but when I'm validating major changes and I have 4-5 stages, it's hard to follow what happens in each of them. It looks like I would have to start special, dedicated instances with the new setting.

[tonistiigi]

Currently, the concurrency limit can only be set on daemon start.