Improved docker

[cmdoret]

This PR fixes and improves the Dockerfile as follows:

• Multi-stage build with cargo chef
  • Cargo-chef caches compiled dependencies in an intermediate layer. This makes docker build fast as long as dependencies do not change, because only the project code must be recompiled.
• Use Google's debian "distroless" image as the base layer for the runner layer. It is a stripped down linux system with libc (no package manager or anything)
  • This brings the image size down to 31MB
  • The image has very few vulnerabilities (trivy finds 13 low and 4 medium CVEs)
• Use an unprivileged user in the container

[cmdoret]

Notes from @martinfontanet :

• Avoid COPY . if you can specify files
• Check if user creation can be simplified