feat: add extract-script job template

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

extract-script:

  image: alpine

  stage: .pre

  script:

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

    - sed -n '/BEGSCRIPT/,/ENDSCRIPT/p' "$GITLAB_CI_FILES" | sed 's/^  //' >> script.sh

    - export LC_ALL=C.UTF-8

  artifacts:

    when: always

    name: extracted template script

    expire_in: 1h

    paths:

      - script.sh

  rules:

    # exclude merge requests

    - if: $CI_MERGE_REQUEST_ID

      when: never

    # enabled if $GITLAB_CI_FILES is set

    - if: $GITLAB_CI_FILES