feat: parameterized container image for script extraction

variables:

  # base image used to extract the shell script part from TBC templates

  TBC_EXTRACT_IMAGE: registry.hub.docker.com/library/alpine

# this job extracts the bash script from the given template(s)

extract-script:

  image: registry.hub.docker.com/library/alpine

  image: $TBC_EXTRACT_IMAGE

  stage: .pre

  script:

    - echo "#!/bin/bash" > script.sh