feat: parameterized container image for script extraction

templates/extract.yml

+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