Dockerfile for consistent environments on any system

[JonasVerbickas]

It would be extremely useful to make it easy to build the library on any system.

I created a Dockerfile based on the official mmsegmentation repo.

After building the docker image I ran training successfully.

# build docker image
docker build -t open-cd .

# run docker image
docker run -it --rm --gpus all -v ./data:/workspace/data

# start training
python tools/train.py configs/changer/changer_ex_r18_512x512_40k_levircd.py --work-dir ./changer_r18_levir_workdir