Replace local docker-buildx.yml with common docker-buildx.yml

[erikrj]

This project should use the upstream docker build located at https://github.com/truemark/github-workflows/blob/main/.github/workflows/docker-buildx.yml instead of the local one provided. If you need an example of how this is done, refer to https://github.com/truemark/aws-cdk-docker/blob/main/.github/workflows/build.yml.