feat: add Dockerfile and Docker usage documentation

[raztud]

Summary

• Add a Dockerfile to enable running Mimicker as a container, useful for integration testing and CI environments
• Add a Docker section to README.md documenting how to build, run with mounted stubs, extend the image, and configure via environment variables

Details

The Dockerfile uses python:3.12-slim and installs the package via pip directly (leveraging the poetry-core build backend), keeping the image small and free of unnecessary tooling.

By default the container starts an empty Mimicker server on port 8080. Users can define their routes by mounting a Python stub file:

docker run -p 8080:8080 -v ./stubs.py:/app/stubs.py mimicker python /app/stubs.py

Or by extending the image with their own Dockerfile.

Test plan

• docker build -t mimicker . - completes successfully
• docker run -p 8080:8080 mimicker - starts the server (returns 404 with no routes configured)
• Mounting a stubs.py with GET/POST routes responds correctly (curl returns expected JSON) - described in README