Update configure_cosign_private_key in gitlab-ci-docker.yml file

Presentation

The issue was caused by missing double quotes in the configure_cosign_private_key() function, specifically in the if -z "${DOCKER_COSIGN_PRIVATE_KEY:-$COSIGN_PRIVATE_KEY}" and if -f "${DOCKER_COSIGN_PRIVATE_KEY:-$COSIGN_PRIVATE_KEY}" conditions.

Fixes #146 (closed) close #146 (closed)

Checklist

• Tested & examplified:
  • With DOCKER_COSIGN_PRIVATE_KEY file
  • With DOCKER_COSIGN_PRIVATE_KEY variable
  • With DOCKER_COSIGN_PRIVATE_KEY variable undefined