Optimize Dockerfile and implement CI/CD to DockerHub

[EONRaider]

This PR fixes and modifies the Dockerfile in GAU with the following considerations:

• The current Dockerfile does not leverage multi-stage builds. The current image has about 868MB with the current Dockerfile instructions. With multi-stage build it has about 16MB.
• Adds deterministic base images for reproducible builds.
• Added a least-privileged user in compliance with Docker security best practices.

We also have a workflow with a CICD pipeline to DockerHub. All you need to do is to add the DOCKER_HUB_USERNAME and DOCKER_HUB_ACCESS_TOKEN secrets to the repository and all pushes/merges to the master branch will automatically push an image to DockerHub.

[lc]

lgtm! thanks for the PR ✌🏼