Service not set when you register docker executor with --docker-services

Overview

Since %12.7 we introduced #4114 (closed) which changes the [[runners]].docker.service from a []string into a struct so when you pass gitlab-runner register --docker-services alpine it used to work but not it's gives you an empty config since it's no longer a string but a struct.

12.6

gitlab-runner register --docker-services alpine:3.11

concurrent = 1
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "xxx"
  url = "http://127.0.0.1:3000/"
  token = "xxxx"
  executor = "docker"
  [runners.custom_build_dir]
  [runners.docker]
    tls_verify = false
    image = "alpine:3.11"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    services = ["alpine:3.11"]               <--------- The service configured
    shm_size = 0
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]

12.7

gitlab-runner register --docker-services alpine:3.11

[[runners]]
  name = "xxx"
  url = "http://127.0.0.1:3000/"
  token = "xxx"
  executor = "docker"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
  [runners.docker]
    tls_verify = false
    image = "alpine:3.11"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0

    [[runners.docker.services]]
      name = ""