Remove unused install.sh

Closes #4192

See merge request gitlab-org/gitlab-runner!1336

Docs: Update contributing links from gitlab-ce

See merge request gitlab-org/gitlab-runner!1308

Fix link syntax in advanced-configuration.md

See merge request gitlab-org/gitlab-runner!1311

Exclude mock files from coverage reporting

See merge request gitlab-org/gitlab-runner!1334

Better explain the workflow in Docker executors doc

Closes #4153

See merge request gitlab-org/gitlab-runner!1310

Runner's codebase contains a lot of mock files. They are used only for
tests and not for the production use.

Unfortunately, because the files are not named as *_test.go - to allow
use the mocks in tests outside of the mock source package - they are
counted to the coverage report.

We're using https://github.com/vektra/mockery to automatically generate
mock files basing on interfaces. Mockery is creating mocks, that
accept both scalar values as well as functions that accept mocked method
argumets and returns mocked method result. However, we're mostly not
using this mocking mechanism.

Finally this ends in a lot of mocks code being not covered in test,
which decreases the overall coverage value. While we're totally not
interested in having coverage for mocks!

This commit aims to remove all mocks from the coverage profile file,
which should remove them from both percentage value calculation and the
HTML/jUnit reports.

Update cmd script example

See merge request gitlab-org/gitlab-runner!1333

Add changelog for v11.9.1 & v11.9.2

See merge request gitlab-org/gitlab-runner!1307

Since gitlab-org/gitlab-runner!1260 we
used delayed expansion for errorlevel when running Windows Batch.

Fix Markdown table not rendering in docs.gitlab.com

See merge request gitlab-org/gitlab-runner!1328

Leave a newline before the table so that it renders correctly.
This is how the current implementation of gitlab-kramdown works.

Change deprecation of FF_USE_LEGACY_BUILDS_DIR_FOR_DOCKER to 12.3

See merge request gitlab-org/gitlab-runner!1330

Refactor helperimage package tests

Closes #4179

See merge request gitlab-org/gitlab-runner!1327

- Use map[string]struct{} for tests with name
- Use consistent naming for table tests
- Use constant for windows supported architecture

closes gitlab-org/gitlab-runner#4179

Fix broken internal links

Closes #4181

See merge request gitlab-org/gitlab-runner!1332

Rename `shells/README.md` to `shells/index.md` to follow the new naming
convention
https://docs.gitlab.com/ee/development/documentation/styleguide.html#working-with-directories-and-files

closes gitlab-org/gitlab-runner#4181

Refactor Helper Image package to work with Kubernetes

See merge request gitlab-org/gitlab-runner!1306

Both Docker and Kubernetes executor are checking what kind of revision
is being used so it can be used to pull in the right helper image,
extract that logic to `helperImage` package.

The Kubernetes executor is using the same logic to build the helper
image tag, which is just duplicated from the docker executor, start
using the helperimage to use the same implementation.

Update feature flag documentation

See merge request gitlab-org/gitlab-runner!1326

Clear up docs on how to select shell

See merge request gitlab-org/gitlab-runner!1209

Applying wording changes from a review for gitlab-org/gitlab-runner!1209

The helper image is being used for both Kubernetes and Docker executor.
But the code does not represent this, instead it duplicates the logic,
such as creating the Linux tag and the name of the image.

Refactor the helperimage package so that it can be used for both, by
removing specific structs from the docker API, by defining helperimage
own Config struct.

The need for the refactor is for
!1243, because
we need to define different commands to run on the helper image, which
is located in `common/container.go` which needs to change depending on
the OS Type of docker, and we can't change it before we change how
Kubernetes uses the helper image.

The feature flags disables the new code path not enabling it, the new
code path is enabled by default.

Remove some whitespace and order struct initialization

Create a single source of truth for feature flags

See merge request gitlab-org/gitlab-runner!1313

The constants are now defined inside of the 'featureflags' package,
so there is no need for the 'FF' prefix. The fact that it's a feature
flag is quite obvious when looking on an entry like:

    IsFeatureFlagOn(featureflags.UseLegacyBuildsDirForDocker)

Allow to use FF to configure `/builds` folder

See merge request gitlab-org/gitlab-runner!1319

Update link to the introduction of custom build directories

See merge request gitlab-org/gitlab-runner!1302

Allow to configure FF using config.toml

See merge request gitlab-org/gitlab-runner!1321