feat: implement backend prototype

[0xpanoramix]

Description

This PR adds a minimal NestJS backend service, with the default route from the official template. It also adds a build system for both dev and production modes using Docker.

Finally, a first version of the CI has been added to run the backend tests.

Changes include

• Bugfix (non-breaking change that solves an issue).
• Hotfix (change that solves an urgent issue, and requires immediate attention).
• New feature (non-breaking change that adds functionality).
• Breaking change (change that is not backwards-compatible and/or changes current functionality).
• Documentation update (change added to any kind of documentation).

Checklist

• I have assigned this PR to myself.
• I have added at least 1 reviewer.
• I have added the relevant labels.
• I have updated the official documentation.
• I have added sufficient documentation in code.

Testing

• I have tested this code with the official test suite.