[demo] add GPU to resources (#355)

This small PR adds GPU specification in `docker-compose.yaml` for the SAM 2 interactive webdemo, following https://docs.docker.com/compose/how-tos/gpu-support/#example-of-a-compose-file-for-running-a-service-with-access-to-1-gpu-device. It fixes a GPU access error as reported in https://github.com/facebookresearch/sam2/issues/354.

(close https://github.com/facebookresearch/sam2/issues/354)