Docker with GitHub actions

[pflooky]

Changes

• Add in a Docker image for the maestro-server
• Build multi-platform images via GitHub Action

Why

• Allow users to deploy and run via Docker images
• Can start Maestro without building (no dependency on Java or Gradle version)

[pflooky]

If you want to try it out now, I've onboarded it into insta-infra (another project I started a while ago). So you can run: ./run.sh maestro