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

This post introduces a meta-practice for GitLab CI/CD: building custom Docker images within the CI pipeline and storing them in the GitLab Container Registry. It details how to create a Dockerfile for build dependencies (e.g., Node.js with Firebase CLI), configure a CI job to build and push this image using `docker:stable` and `docker:dind`, and then use this custom image as the base for subsequent deployment jobs. The `only: changes` keyword is highlighted for triggering image rebuilds only when the Dockerfile is modified.