Switch to Docker Hardened Images (DHI) for base images in `Dockerfile`

We use Label Studio's official Docker images as a base (via FROM) and apply some modifications on top. However, we keep running into vulnerabilities when we scan these images.

I believe the root cause is that Label Studio's Docker image uses the standard python image from Docker Hub as its base. Switching to [Docker Hardened Images](https://www.docker.com/products/hardened-images/) (DHI) instead would likely resolve these vulnerabilities. DHI are now free and open source, released under the Apache 2.0 license.

I'd be happy to open a Pull Request to make this change, but there's one thing to be aware of: pulling DHI requires authentication with a Docker Hub account, so you would need to configure that on your end as well.