onnxruntime

Allow building Docker container based on a different git repo. (#1222)

- Introduce Docker build ARG `ONNXRUNTIME_REPO`
  to allow building Docker container based on a different git repo.

Example docker build command:

```bash
cd dockerfiles
docker build -t onnx-runtime \
  --build-arg ONNXRUNTIME_REPO=https://github.com/jthelin/onnxruntime \
  --build-arg ONNXRUNTIME_SERVER_BRANCH=my-branch \
  -f Dockerfile.server .
```

- Add a basic `.dockerignore` file, to cut down the number of filles passed into the Docker build context.