Parameterise runtime image (#1478)

* Use distroless debian11 docker image

* Add `Dockerfile` to `.dockerignore`

* Replace `nonroot` with the matching UID/GID
Alpine does not have that user, and it cause issues when trying to start the container

* Use a build arg for setting the runtime image

* Explain why `ARG RUNTIME_IMAGE` is at the top

* Add entry to CHANGELOG

* Move build-arg to `DOCKER_BUILDX_ARGS`