Add GHA CI & Docker

[DavidPL1]

Adds GitHub Actions for creation of Docker images with pre-installed MuJoCo versions, and CI build and testing with CodeCov

Todos

• Github Apps:
  • [ ] Install behavior bot for this repo (https://github.com/behaviorbot/welcome)
  • [ ] Install CodeCov for this repo (https://github.com/apps/codecov)
• Docker
  • [ ] Change dockerhub user
  • [ ] Add dockerhub secrets
• Formatting
  • [ ] Apply formatting result patch to PRs (?)
• Docs
  • [ ] Change README.md badge links

[rhaschke]

I addressed my comments and created a new PR using the add-gha branch in this repo.