Add support for private networks

[href]

To test, use the following image in your config for gitlab_runner.fleeting_image:

# GitLab runner instances settings
gitlab_runner:

  # The fleeting plugin image to use
  fleeting_image: quay.io/cloudscalech/fleeting-plugin-cloudscale-beta:latest

You should then be able to create setups where the GitLab Runner communicates via private network:

gitlab:
  networks:
    - public
    - <private-network-uuid>
 
gitlab_runner:
  networks:
    - <private-network-uuid>
    - public

A fully private network should work as well, though you would need a NAT gateway in that case, as the GitLab runner needs internet access.

To test if a job runs, you can create a project with the following .gitlab-ci.yml, which is what I usually use:

image: debian

cache:
  key: global-cache

build:
  script:
    - apt-get update
    - apt-get install -y cowsay
    - >
      test -f cached
      && /usr/games/cowsay "I ran on $CI_RUNNER_DESCRIPTION and found a cache"
      || /usr/games/cowsay "I ran on $CI_RUNNER_DESCRIPTION and found no cache"
    - touch cached
    
  cache:
    paths:
      - cached

[k-304]

LGTM. Added notes about minor things I noticed.