How to supply custom options for gitlab-runner's config file

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In nixos 20.09, I would like to add custom options to the config.toml used to configure gitlab runners. I want to do this because the options I need to add don’t seem to be supported by nixos.

There is an option called services.gitlab-runner.configFile but it is deprecated.

Here is the bit I need to add to the config file:

  [runners.cache]
    Type = "s3"
    Path = "runner-cache"
    Shared = true
    [runners.cache.s3]
      ServerAddress = "..."
      AccessKey = "..."
      SecretKey = "..."
      BucketName = "..."
      Insecure = true

Any ideas?