Merge branch '6389-service-not-set-when-you-register-docker-executor-with-docker-services' into 'master'

Fix docker services regression flag

Closes #6389

See merge request gitlab-org/gitlab-runner!1776

Allow service alias from config in Docker executor

Closes #4114

See merge request gitlab-org/gitlab-runner!1673

Copyedit doc for volumes_from in Docker executor config

See merge request gitlab-org/gitlab-runner!1750

Refactor metrics referee

See merge request gitlab-org/gitlab-runner!1725

The `_referee` is redundant since the file is under the `referees`
directory.

`uploadReferee` always return a `nil` error, so there is no need to
return anything.

`CreateMetricsReferee` does not follow Go idioms where the `new` keyword
should be used when there is a constracutor of a struct that returns a
pointer.

Make this function internal and not exposed since a the moment the only
entry point of the `referee` package should be `CreateReferees`

Refactor metrics referee tests

See merge request gitlab-org/gitlab-runner!1714

Change to go1.13 error wrapping

See merge request gitlab-org/gitlab-runner!1709

Make 'tags: gitlab-org' a re-usable job tempalte

See merge request gitlab-org/gitlab-runner!1745

docs: Replace ruby:2.1/2.2 with ruby:2.6 or 2.7

See merge request gitlab-org/gitlab-runner!1748

Add missing 'needs' entry for release docker images job template

See merge request gitlab-org/gitlab-runner!1744

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Ruby 2.1/2.2 have been deprecated (EOL-ed) in 2018 or earlier.

Example docker image version has been updated from 2.1 to 2.6:
https://gitlab.com/gitlab-org/gitlab-runner/merge_requests/1391



Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Refactor CI pipeline

See merge request gitlab-org/gitlab-runner!1733

Do not embed mutex

See merge request gitlab-org/gitlab-runner!1734

Remove some duplication on setting up the test.

Run https://github.com/fatih/errwrap on Runner source code `errwrap -fix
./...`. This transform err handing to the new go1.13 syntax
https://blog.golang.org/go1.13-errors

Windows likes backslashes, not forward ones....

See merge request gitlab-org/gitlab-runner!1738

Add a git version caveat

See merge request gitlab-org/gitlab-runner!1732

Embedding exposes methods of the embedded type. Mutex
is an implementation detail and it's methods should not
be exposed.

Fix Typos

See merge request gitlab-org/gitlab-runner!1731

Update advanced-configuration.md - fix typo of mperiods

See merge request gitlab-org/gitlab-runner!1722

* 12-6-stable:
  Update CHANGELOG for v12.6.0