support cross-platform docker images

[raohuaming]

Support using buildx to build cross-platform images:

$ docker buildx create --name clearml-server --use
$ docker buildx build -f docker/build/Dockerfile --platform linux/arm64,linux/amd64 -t  raohuaming/clearml:v1.6.0 --push .

[jkhenning]

Hi @raohuaming ,

Thanks for this contribution. I'm not sure I understand how to build for cross-platform, can you please explain?
In general, I would prefer to:

• Have an additional build target/script/Dockerfiles for this cross-platform build and keep the original build as is
• Since the server repository is used for development and not only for deployment (mostly for development), I would prefer not to use git submodules as we prefer to use these for code submodules that are required for development, not deployment.