GitLab environment variables demystified

What this post added

This post provides a comprehensive overview of GitLab CI/CD variables, detailing their definition, scope, and precedence. It explains how variables can be defined in `.gitlab-ci.yml`, at the project level, group level, and instance level. It also covers predefined variables and custom variable types (Type and File), along with configuration options like Environment scope, Protected variables, Masked variables, and the newer Masked and hidden variables. The post also touches upon secrets management, including external secrets integration and the upcoming native GitLab Secrets Manager.