Support overriding docker-daemon image on self-hosted runners

Hi team,

Thanks for your efforts that PAUSE_IMAGE, AUTH_PROXY_IMAGE, and CLONE_IMAGE are provided to support custom Docker registry in the self-hosted runner. Is it possible to support docker-daemon image (ie. docker-public.packages.atlassian.com/sox/atlassian/bitbucket-pipelines-docker-daemon:v20.10.18-prod-stable) as well?

Here are the use cases:
(1) The firewall blocks the traffic between our internal environment and docker-public.packages.atlassian.com. So we have to use an internal docker registry for remote docker registry mirror/cache and override the docker image on our internal registry mirror.

(2) We has built our custom docker daemon for most cases in our internal environment. At the moment, for each pipeline which has enabled docker service, we have to define the following docker service in the pipeline.yaml. If overriding docker-daemon image is supported, we may configure our <internal-docker-registry>/<custom-bitbucket-pipelines-docker-daemon> as the default docker daemon, so we could remove the following code from each pipeline and therefore the pipeline.yaml becomes simpler and easier to understand for our developers.

```
definitions:
  services:
    docker:
      image:
        name: <internal-docker-registry>/<custom-bitbucket-pipelines-docker-daemon>
      environment:
        HTTP_PROXY: $HTTP_PROXY
        HTTPS_PROXY: $HTTP_PROXY
        NO_PROXY: $NO_PROXY
```

Kind regards,
Aaron