Allow optional runner register params, bump machine version

[sammcj]

• Allow for passing in optional gitlab-runner register parameters as an environment variable.
  • This should allow for setting docker-machine to run on the private IP rather than public by passing something like amazonec2-use-private-address=true which then allows you to lock down the security group and mitigates https://github.com/woodjme/autoscaling-ec2-gitlab-runners-fargate/issues/2
  • You could also pass in options such as --tag-list mytag to register the runner with a specific ci tag. (fixes https://github.com/woodjme/autoscaling-ec2-gitlab-runners-fargate/issues/1)
• Bump docker-machine version to latest stable (v0.16.2)

[woodjme]

Hey @sammcj

If you could rename RUNNER_REGISTER_PARAMS to ADDITIONAL_REGISTER_PARAMS I'd be happy to merge. I think it just helps clarify that there are some params already configured in the image.

For now, remove the changes to the readme and the default --machine-machine-options amazonec2-use-private-address=true as I've got some proposed changes over at #2

[sammcj]

Hi @woodjme,

Thanks for reviewing this, I've made your suggested changes to the MR.