Add CUDA support to the Dockerfile

[lmoe]

Hey,

this is one out of two PR's. I hope you find them beneficial.

I added optional GPU support to the Dockerfile. The current image only works with CPU. With --build-arg GPU=1 it uses a CUDA base image and pulls the CUDA torch wheels.

This enables --device cuda for the node.

# CPU (default, same as before)
docker build -t wyoming-faster-whisper .

# GPU
docker build --build-arg GPU=1 -t wyoming-faster-whisper:gpu .

The way this works is a bit ugly (Docker doesn't have proper conditionals for FROM), but it keeps the CPU images slim.

The downside is that the build will download the CUDA base image even when building for CPU, but it won't be included in the final result. Since these are built on CI anyway, I wouldn't mind this much personally.

Default is GPU=0 so nothing breaks for existing builds.

(I chose cu126 because it still supports older GPU's such as the 1080. Newer wheels would exclude them, but they still perform quite good with Whisper.)

Cheers