Add docker containers validation in CI

[Kavignon]

Following up on the work done here: #9

I want to extend what's already established in the CI by validating the Docker containerization setup built through #13.

During this step, I want to perform the following (at a minimum):

• Build the image for Slax
• Run the application within the container with all its required services (Postgres)
• Run the test suite from the container
• Check if we can access http://localhost:4000 from the container
• Clean up the containers, files, and any other artifacts created during this stage.