1. Remove conda from the images. Because conda contains a file named /opt/miniconda/lib/libcrypto.so.1.0.0 which can't pass our security scan. Also, it will be easier for us to manage the third party usage registrations.
2. Remove openssh from the images. Because the official openssh package provided by Ubuntu can't pass our security scan.
3. Reduce the image size to 1/3 by using stages. Also, because it contains less packages, it will be less often needed to update.
4. Put the LICENSE-IMAGE.txt file in right place. It is missed in current images. You can see it was added to a temp folder "/code" but it got deleted afterwards.
5. Update the CPU docker image's base image to Ubuntu 18.04. The GPU one is already 18.04. It's better to keep them the same.
6. Remove the build arg ONNXRUNTIME_REPO/ONNXRUNTIME_BRANCH. Instead, the new one always uses the local source. I feel it can reduce confusion.
- 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.
