What's the use of the [runners.docker] section in config.toml for use case with docker machine? - gitlab-ci

Question

reading the documentation on autoscaling I can't figure the role of the [runner.docker] section when using docker+machine as executor :
[runners.docker]
image = "ruby:2.1" # The default image used for builds is 'ruby:2.1'
In the executors documentation it says :
docker+machine : like docker, but uses auto-scaled docker machines -
this requires the presence of [runners.docker] and [runners.machine]
I get I have to define this [runners.docker] section to be able to use [runners.machine] section, but what is the aim of this [runners.docker] ?
I can't find how to configure it as I don't understand why to use it.
Our gitlab-runner runs on a vSphere VM and is configured to scale using docker+machine executor with MachineDriver using vmwarevsphere. All works nice but I would like to understand fully the configuration file.
Here is our "censored with stars" config.toml file with the [runners.docker] I can't understand (note that the guy that wrote it leaved the company, so I can't ask him):
[[runners]]
name = "gitlab-runner"
limit = 6
output_limit = 102400
url = "http://gitlab.**************.lan"
token = "*******************"
executor = "docker+machine"
[runners.docker]
tls_verify = false
image = "docker:latest"
dns = ["*.*.*.*"]
privileged = true
disable_cache = false
volumes = ["/etc/localtime:/etc/localtime:ro", "/var/run/docker.sock:/var/run/docker.sock", "/etc/docker/certs.d:/etc/docker/certs.d", "/cache:/cache", "/builds:/builds"]
cache_dir = "cache"
shm_size = 0
[runners.cache]
Type = "s3"
ServerAddress = "*.*.*.*"
AccessKey = "*****************"
SecretKey = "*****************"
BucketName = "runner"
Insecure = true
[runners.machine]
IdleCount = 4
MaxBuilds = 10
IdleTime = 3600
MachineDriver = "vmwarevsphere"
MachineName = "gitlab-runner-pool-1-%s"
MachineOptions = ["vmwarevsphere-username=************", "vmwarevsphere-password=*****************", "vmwarevsphere-vcenter=*.*.*.*", "vmwarevsphere-datastore=*********", "vmwarevsphere-memory-size=3096", "vmwarevsphere-disk-size=40960", "vmwarevsphere-cpu-count=3", "vmwarevsphere-network=*****************", "vmwarevsphere-datacenter=**************", "vmwarevsphere-hostsystem=*******************", "engine-storage-driver=overlay2", "engine-insecure-registry=**************", "engine-insecure-registry=*******************"]
OffPeakPeriods = ["* * 0-8,21-23 * * mon-fri *", "* * * * * sat,sun *"]
OffPeakTimezone = "Local"
OffPeakIdleCount = 1
OffPeakIdleTime = 600

Answer 1

The [runners.machine] section defines how to start and provision your runner machines, the [runners.docker] section then defines how to configure the runner on that machine.
Docker-machine on its own only does the following (as you can read here):
"Docker Machine is a tool that lets you install Docker Engine on virtual hosts, and manage the hosts with docker-machine commands."
So this does nothing with the Gitlab runner, you still need to configure the runner after that and thats where the [runners.docker] section comes into play because the runner needs to know what default image to use and what volumes to mount etc.