Support selectable number of CPUs for docker_builder

[asomers]

Description

The docker_builder task should support a selectable number of CPUs. Right now it's hard-coded to 4.

Context

The Nix project has some tasks that must use docker_builder, even though they can only take advantage of a single CPU. So we're wasting resources. Our builds would schedule sooner, and your bills would be lower, if we could reduce the CPU count.

Anything Else

The task in question:
https://github.com/nix-rust/nix/blob/5729d0f391775c98699f0f92f8df6b7f5f49fc7d/.cirrus.yml#L53

[RDIL]

Good idea! Can't hurt to have faster build times 😄

[asomers]

I tried following the instructions in 25346a5 , but they don't work. My builds failed with a Failed to start an instance: INVALID_ARGUMENT: Forbidden error. Is something about this not allowed for community cluster builds? Here is the relevant excerpt of my .cirrus.yml:

task:
  env:
    ...
  matrix:
    ...
  compute_engine_instance:
    image_project: cirrus-images
    image: family/docker_builder
    platform: linux
    cpu: 1                          # Since QEMU will only use 1 thread
    memory: 4G
  setup_script:
    ...

nix-rust/nix#1392

[fkorotkov]

@asomers should be docker-builder with a dash and not underscore _.