Allow overriding DOCKER_DRIVER in Auto DevOps Template

Summary

The current implicit CI configuration for Auto DevOps sets the variable DOCKER_DRIVER to overlay2, which is a sane choice for many environments. There is no documented way (and I cannot think of any undocumented one) to override this and in our environment it fails, since we need to use the zfs driver.

Steps to reproduce

• use docker with zfs driver
• configure gitlab-runner with docker or kubernetes executor targeting the zfs docker
• watch how everything using docker:dind in an Auto DevOps pipeline fails
  • docker:dind does not start successfully

Example Project

We use 11.4.5 locally and I do not think an example project would add more information here.

What is the current bug behavior?

The variable DOCKER_DRIVER is set to a static value and cannot be overriden.

What is the expected correct behavior?

The variable DOCKER_DRIVER is set to a sane default (overlay2) and there is a documented way to override it per instance or, even better, per gitlab-runner.

Relevant logs and/or screenshots

Output of a failing job

Running with gitlab-runner 11.4.2 (cf91d5e1)
  on Kubernetes Runner a2ef97b9
Using Kubernetes namespace: gitlab-runner
Using Kubernetes executor with image docker:stable-git ...
Waiting for pod gitlab-runner/runner-a2ef97b9-project-287-concurrent-0w6ftj to be running, status is Pending
Waiting for pod gitlab-runner/runner-a2ef97b9-project-287-concurrent-0w6ftj to be running, status is Pending
Waiting for pod gitlab-runner/runner-a2ef97b9-project-287-concurrent-0w6ftj to be running, status is Pending
Waiting for pod gitlab-runner/runner-a2ef97b9-project-287-concurrent-0w6ftj to be running, status is Pending
Running on runner-a2ef97b9-project-287-concurrent-0w6ftj via gitlab-runner-6b5cb48957-n7db5...
Cloning repository...
Cloning into '/mgr/deobfuscate-ws'...
Checking out 3b31e02a as master...
Skipping Git submodules setup
$ # Auto DevOps variables and functions # collapsed multi-line command
$ setup_docker
$ build
Logging to GitLab Container Registry with CI credentials...
WARNING! Using --password via the CLI is insecure. Use --password-stdin.
Warning: failed to get default registry endpoint from daemon (Cannot connect to the Docker daemon at tcp://localhost:2375. Is the docker daemon running?). Using system default: https://index.docker.io/v1/
Cannot connect to the Docker daemon at tcp://localhost:2375. Is the docker daemon running?
ERROR: Job failed: command terminated with exit code 1

Output of checks

Results of GitLab environment info

# gitlab-rake gitlab:env:info

System information
System:		Debian 9.5
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.4.5p335
Gem Version:	2.7.6
Bundler Version:1.16.2
Rake Version:	12.3.1
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.1
Go Version:	go1.7.4 linux/amd64

GitLab information
Version:	11.4.5-ee
Revision:	bf6db46
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
DB Version:	9.6.8
URL:		https://gitlab.autinity.de
HTTP Clone URL:	https://gitlab.autinity.de/some-group/some-project.git
SSH Clone URL:	git@gitlab:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: autinity, github

GitLab Shell
Version:	8.3.3
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks
Git:		/opt/gitlab/embedded/bin/git

Possible fixes

Line of code causing the issue: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml#L55

I don't know what the best fix would be. Only idea I have is renaming the variable to DOCKER_DRIVER_DEFAULT at that line and checking in every job if DOCKER_DRIVER is set and when it isn't, set it to DOCKER_DRIVER_DEFAULT.