feat: add amazonec2 userdata for docker machines

[FelikZ]

Description

Introducing support for amazonec2-userdata flag that is not mentioned on driver readme page, but it is there and useful.

For example, it can be used to introduce swap for the runner instance or any other user command.

Migrations required

NO - this is optional flag and defaults to empty string.

Verification

I have verified on my own runners cluster that user script that I've added works (it creates swap in my case).

module "runner" {
  # source  = "npalm/gitlab-runner/aws"
  # version = "5.5.0"

  # ...

  runners_userdata = templatefile("${path.module}/../../templates/swap.tpl", {
    swap_size = "512"
  })
}

[FelikZ]

But I have no idea how to auto-generate README.md, I think this flag should be documented. (manually edit?)
UPD: I see there is github action for that CI / Auto update terraform docs (pull_request), neat.

[kayman-mk]

Good PR! Just 2 little remarks.

[kayman-mk]

@npalm See my comment above. Better you check this as well.

[npalm]

@FelikZ thanks looks good! Please can you add the example. Which you mention in the PR to either the Readme. Or as comment in the default example. I would prefer the last one (example)

[FelikZ]

@npalm please have a look. Thanks

[kayman-mk]

Did you do a quick check without using the new variable runners_userdata to make sure that the agent is still able to start?

changes applied

[kayman-mk]

Ok, it's working. Had a test environment available and deployed your changes. It is working without runners_userdata.