Add Category Based Sorting Functionality (GitLab Duo)

This code change reorganizes the project structure by moving product-related code from one file to another. Specifically, it moves the Product class definition and all the sample product data (like GitLab t-shirts, beanies, mugs, etc.) from a file called "models.py" into the "db.py" file. The main application file is updated to import these items from their new location. Additionally, the Docker configuration is updated to use a newer, more standard base image. The functionality remains exactly the same - it's purely a reorganization to better group database-related code together in one place.