Using docker executor instead of shell

What does this MR do and why?

Attempting to fix https://gitlab.com/gitlab-org/gitlab/-/issues/456895#note_1917747951 - where the job gets stuck in a loop of bash command error docker: command not found.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Run test against Staging

GITLAB_USERNAME=<username> GITLAB_PASSWORD=<password> GITLAB_QA_USER_AGENT=<user_agent> GITLAB_QA_ACCESS_TOKEN=<token> QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/api/5_package/container_registry/saas/container_registry_spec.rb

Related to #456895