Merge branch 'patch-3' into 'master'

Fix docker.sock reference in config.toml

## What does this MR do?

`config.toml` sample did not correctly represent the `/var/run/docker.sock` mapping, causing it to be created as a volume instead.

## Why was this MR needed?

It was broken.

See merge request !5172

doc/ci/docker/using_docker_build.md

@@ -188,7 +188,7 @@ In order to do that, follow the steps:
         image = "docker:latest"
         privileged = false
         disable_cache = false
-        volumes = ["/var/run/docker.sock", "/cache"]
+        volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
       [runners.cache]
         Insecure = false
     ```