Set service entrypoint and command through config

Problem

Inside of the config.toml we allow the user to define services for the Docker executor and Kubernets executor. If we look at all the allowed configuration for the services inside of the .gitlab-ci.yml we are missing the entrypoint and command fields.

Proposal

• Extend the service 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

[[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"]