Change the dockerfile from tiangolo/gunicorn to python3.11

[kj3moraes]

The base Docker image that is being used for the backend has been marked as deprecated - See here. We should switch to writing our own Docker file from scratch.

Reference Links

1. FastAPI documentation about: FastAPI in Containers - Docker.
2. FastAPI in Containers - Docker: Build a Docker Image for FastAPI.
3. FastAPI Docs about Deployment with Docker.