Update Dockerfile to use python:3.6-buster and fix dependencies

[loopassembly]

• Change base image from python:3.6-stretch to python:3.6-buster to avoid outdated package repositories causing build failures.
• Add installation of numba==0.48.0 and resampy==0.2.2 to resolve ImportError and TypeError issues related to librosa and resampy.
• Ensure compatibility with updated dependencies and improve build reliability.

These changes address the following issues encountered during the Docker build process:

• The python:3.6-stretch image references outdated Debian Stretch repositories, causing apt-get update failures.
• Missing numba.decorators module required by librosa.
• TypeError: guvectorize() missing 1 required positional argument: 'signature' caused by an incompatible resampy version.

By switching to python:3.6-buster and explicitly installing numba and resampy, Docker image builds successfully and the application runs without import errors.