GitLab

Various updates for rav1e Docker image

See merge request !1

- Build for both the latest stable and the minimum supported rustc
- Include dav1d, cargo-kcov, and rustfmt
- Refactor Dockerfile into stages for caching and image size optimization
- Use variables for versions of packages we install manually
- Build the Docker image when opening an MR in this repository

Needed for kcov

This is needed for kcov, without it kcov just silently does not build
the kcov binary.