Add Markdown linting and one rule

See merge request !1422

* 12-0-stable:
  Update CHANGELOG for v12.0.0
  Set proper version info in VERSION file

Add missing test cases for s3 IAM checks

See merge request !1421

Wrap submodule command with a string

Closes #4352

See merge request !1411

ShouldUseIAMCredentials checks if any of the fields are empty if one of
them is empty it will return true.

Most test cases are missing, the only thing tested is when all fields
are empty.

In
!1365 (comment 183850614)
a change was made that would have broken the check but CI did not catch
this.

Replace wget commands with curl commands

Closes #4345

See merge request !1419

Add tip to execute batch from PowerShell

See merge request !1412

Upgrade base image for gitlab/gitlab-runner:ubuntu to ubuntu:18.04

Closes #3954

See merge request !1413

Fix Powershell scripts for Docker containers

See merge request gitlab-org/gitlab-runner!1414

Multi-line Powershell commands have to be closed by an empty line
if piped to a Docker container.
Otherwise the code block is not executed.

gitlab-org/gitlab-runner#4291

Some packages (e.g. tzdata which was direct cause of this change)
normally provide a UI for configuration. This however doesn't work
for automated usages like building a Docker image.

This commit configures packaging system to behave as an
noninteractive process.

Upgrading to Git 2.22, breaks the scenario when you have a Git
repository with submodules and `GIT_SUBMODULE_STRATEGY` is set to
`recursive`.

The command `git submodule foreach --recursive git clean -ffxd` is build
and this works fine in Git <2.21 but in Git 2.22 it fails with the
following error `error: unknown switch `f'` the reason for this is
because Git seems to be thinking that `-ffxd` is a flag for `git
submodule` and not for `git clean`. Wrapping the commands with single
quotes to fix this issue.

Fix "WARNING: apt does not have a stable CLI interface. Use with caution in scripts"

See merge request !1143

Update logrus to v1.4.0

See merge request !1407

https://github.com/sirupsen/logrus/releases

Restore gofmt rules from before codeclimate update

See merge request !1408

Make PowerShell default for new registered Windows shell executors

See merge request !1406

Update osx.md so the update instructions work as well as the install instructions

See merge request !1402

When a user registers a new Shell executor `cmd` is the default shell.
In 11.11 Windows Batch was deprecated
https://about.gitlab.com/2019/05/22/gitlab-11-11-released/#support-for-windows-batch-is-now-deprecated.
For new registered Runners Powershell should be the default shell used
to increase adoption of PowerShell, the user still has the option to go
back to `cmd` if they specify it.

Reference #4163

Update some logrus fields used in Runner logs

See merge request !1405

Numerous typos were fixed. Some unused constants, variables and arguments were removed.

See merge request !1258

Docker Credentials helper support

See merge request !1386

Update compatibility heading as it's no longer a chart/table

See merge request !1401

Maintainer

Update github.com/Microsoft/go-winio dependency

Closes #2860

See merge request !1348

Fixed date typo

See merge request !1394

Fix default cache volume docker-windows register

Closes #3915

See merge request !1391