Remove support for --docker-services flag on register command

Related to https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/2036

doc/ci/docker/using_docker_images.md

@@ -26,7 +26,20 @@ test them on a dedicated CI server.
 To use GitLab Runner with Docker you need to [register a new Runner](https://docs.gitlab.com/runner/register/)
 to use the `docker` executor.
 
-A one-line example can be seen below:
+An example can be seen below. First we set up a temporary template to supply the services:
+
+```shell
+cat > /tmp/test-config.template.toml << EOF
+[[runners]]
+[runners.docker]
+[[runners.docker.services]]
+name = "postgres:latest"
+[[runners.docker.services]]
+name = "mysql:latest"
+EOF
+```
+
+Then we register the runner using the template that was just created:
 
 ```shell
 sudo gitlab-runner register \
@@ -34,9 +47,8 @@ sudo gitlab-runner register \
   --registration-token "PROJECT_REGISTRATION_TOKEN" \
   --description "docker-ruby:2.6" \
   --executor "docker" \
-  --docker-image ruby:2.6 \
-  --docker-services postgres:latest \
-  --docker-services mysql:latest
+  --template-config /tmp/test-config.template.toml \
+  --docker-image ruby:2.6
 ```
 
 The registered runner will use the `ruby:2.6` Docker image and will run two

doc/ci/examples/test-and-deploy-python-application-to-heroku.md

@@ -76,14 +76,21 @@ To build this project you also need to have [GitLab Runner](https://docs.gitlab.
 You can use public runners available on `gitlab.com` or you can register your own:
 
 ```shell
+cat > /tmp/test-config.template.toml << EOF
+[[runners]]
+[runners.docker]
+[[runners.docker.services]]
+name = "postgres:latest"
+EOF
+
 gitlab-runner register \
   --non-interactive \
   --url "https://gitlab.com/" \
   --registration-token "PROJECT_REGISTRATION_TOKEN" \
   --description "python-3.5" \
   --executor "docker" \
-  --docker-image python:3.5 \
-  --docker-services postgres:latest
+  --template-config /tmp/test-config.template.toml \
+  --docker-image python:3.5
 ```
 
 With the command above, you create a runner that uses the [`python:3.5`](https://hub.docker.com/_/python) image and uses a [PostgreSQL](https://hub.docker.com/_/postgres) database.

doc/ci/examples/test-and-deploy-ruby-application-to-heroku.md

@@ -64,7 +64,19 @@ You can do this through the [Heroku Dashboard](https://dashboard.heroku.com/).
 First install [Docker Engine](https://docs.docker.com/installation/).
 
 To build this project you also need to have [GitLab Runner](https://docs.gitlab.com/runner/).
-You can use public runners available on `gitlab.com` or register your own:
+You can use public runners available on `gitlab.com` or register your own. Start by
+creating a template configuration file in order to pass complex configuration:
+
+```shell
+cat > /tmp/test-config.template.toml << EOF
+[[runners]]
+[runners.docker]
+[[runners.docker.services]]
+name = "mysql:latest"
+EOF
+```
+
+Finally, register the runner, passing the newly-created template configuration file:
 
 ```shell
 gitlab-runner register \
@@ -73,8 +85,8 @@ gitlab-runner register \
   --registration-token "PROJECT_REGISTRATION_TOKEN" \
   --description "ruby:2.6" \
   --executor "docker" \
-  --docker-image ruby:2.6 \
-  --docker-services latest
+  --template-config /tmp/test-config.template.toml \
+  --docker-image ruby:2.6
 ```
 
 With the command above, you create a Runner that uses the [`ruby:2.6`](https://hub.docker.com/_/ruby) image and uses a [PostgreSQL](https://hub.docker.com/_/postgres) database.