Set service entrypoint and command through config
## Problem Inside of the [`config.toml`](https://docs.gitlab.com/runner/configuration/advanced-configuration.html) we allow the user to define `services` for the [Docker executor](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runnersdockerservices-section) and [Kubernets executor](https://docs.gitlab.com/runner/executors/kubernetes.html#using-services). If we look at all the [allowed configuration for the `services` inside of the `.gitlab-ci.yml`](https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#available-settings-for-services) we are missing the `entrypoint` and `command` fields. ## Proposal - Extend the [service](https://gitlab.com/gitlab-org/gitlab-runner/-/blob/ebcdf469ec52d4b3d1c75928fca99f88f9a6642d/common/config.go#L356-359) configuration to allow to set `entrypoint` and `command` for both the Docker and Kubernetes executor. - Update documentation to reflect these changes. ## Use cases ### Be able to set the DockerHub mirror for the `dind` service ```yaml [[runners]] [runners.docker] [[runners.docker.services]] name = "docker:19.03.13-dind" entrypoint = ["dockerd-entrypoint.sh"] command = ["--registry-mirror=https://docker-mirror.mycompany.com:5000"] ```
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