Enable `[runner.custom]` configuration via `extra_configs`

[saily]

Enable [runner.custom] configuration via extra_configs in toml template. Currently only [runner.docker] seems to be supported.

See: https://github.com/riemers/ansible-gitlab-runner/blob/master/templates/config.toml.j2#L133-L153

[guenhter]

/cc @LoniasGR

[LoniasGR]

Hi, @saily thanks for your inputs. This should be fairly easy to add. Check out how it was done in: 274f65d

I could also probably take a look, but this will take a bit longer to happen.

Edit: Also leaving the documentation for reference (https://docs.gitlab.com/runner/configuration/advanced-configuration/#the-runnerscustom-section)

[saily]

I'm already on it, you'll receive a PR quite soon.

[guenhter]

@LoniasGR are you interested getting a maintainer of this repo?

[LoniasGR]

Yes, I would be interested for that. I'm not sure I can commit much time, but as far as I see there are no big needs as well.

[guenhter]

/cc @riemers could you give @LoniasGR elevated rights as I have please :)