CI: Moving Docker push script block from YAML to POSIX shell script

GitLab CI/CD configuration improvement

Related to #1397 (closed)

Moving Docker related script blocks from YAML to POSIX shell script files:

• Avoid YAML leaks from special characters
• Allow safer execution with options: exit on error or unset variable (set -eu)

Part 3: Docker push images

Follow-up of !4394 (merged) and !4404 (merged)

Testing

The pipeline is the same, you can build Docker images manually, it will call the template .build_docker_release_template

Automatically triggered with ci--docker label: https://gitlab.com/tezos/tezos/-/jobs/2083896138