Optimize CI with a Custom Docker Image for QGIS Dependencies

[zacdezgeo]

To reduce CI build times and improve reliability, we should either create a custom Docker image preloaded with the QGIS dependencies required for our project or leverage an existing QGIS image if it meets our needs. This will eliminate the overhead of installing QGIS and related libraries during every workflow run, ensuring faster and more consistent builds. The custom image can be hosted on a container registry (e.g., GitHub Container Registry or Docker Hub) and updated as needed when dependencies change.