actually cut to 63 bytes, not characters

variables.sh

 # Set the docker image tag to the name of the current branch.
 # Shortened to 63 bytes because Gitlab does the same with the `CI_COMMIT_REF_SLUG`
 # variable, which we use in the CI script to build the docker image.
-dockerTag=$(git rev-parse --abbrev-ref HEAD | cut -c "-63")
+dockerTag=$(git rev-parse --abbrev-ref HEAD | cut -b "-63")
 
 # Only get the number from the issue (or whatever is in front of a dash
 # otherwise) for the release name