Getting [meta] with GitLab CI/CD: Building build images

What this post added

This post introduces the concept and implementation of building custom Docker 'build images' as a meta-step within GitLab CI/CD pipelines. It details how to use the `docker:stable` image with `docker:dind` service to build a Docker image containing specific dependencies (e.g., `firebase-tools`) and push it to the GitLab Container Registry. The post demonstrates how subsequent jobs can then use this custom image as their base, reducing pipeline execution time and dependency on external Docker Hub images. It also highlights the use of built-in CI/CD variables for registry authentication and the `only: changes` keyword for conditional image rebuilding.