How to limit CPUs used by an implicit Docker builder?

[Minoru]

I use a shorthand syntax:

task:
    name: Example
    container:
        dockerfile: my/example.dockerfile
        cpu: 1
        memory: 512MB
    script:
        - echo 'Cirrus CI is awesome!'

which implicitly creates a task to build an image from my Dockerfile, push it to the private registry at Cirrus, then instantiates a container and runs the script there. The container's CPU and memory are limited according to the above config.

However, the implicit task that builds the Docker image is not limited, and uses 4 CPUs and 15 GB memory. Most of those resources are going unused, wasting my compute credits.

How do I add CPU limits for the implicit Docker builder?