Create user as a system account.

This is an account for a service rather than a human being and should therefore be created as a system user. This puts it in the system UID and GID range.

See merge request !114

Use different detection mechanism



See merge request !127

It should fix all problems with bad `ENTRYPOINT` for Docker Images

Rename links to autoscaling (one word)

Links were broken so I changed them to match the filename

See merge request !125

Rename doc to autoscaling (one word)



See merge request !123

[ci skip]

Update the OSX service instruction

This tries to answer and notice most common problems when using Runner as an service on OSX.

We should add to a blog post information about this findings and advise any OSX user to upgrade startup script:
```
gitlab-runner uninstall
gitlab-runner install
gitlab-runner start
```

The binary:
* https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/osx-launch-agents/binaries/gitlab-ci-multi-runner-darwin-amd64

@axil Could you review?

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/972 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/217 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1143 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/122

See merge request !121

[ci skip]

Major documentation refactor

This tries to fill almost all of the gaps in current runner documentation.

cc @axil @tmaczukin 

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/994


See merge request !79

[ci skip]

[ci skip]

[ci skip]

docs: cache_dir needs to appear in docker's volumes

As found in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/970, a custom `cache_dir` has to be added to `volumes`.

See merge request !120

[ci skip]