* build(ci): added rclone 1.57.0 to Docker container images
/bin/rclone is configured to use config in /app/rclone/rclone.conf
so the user can pass -v ~/.config/rclone:/app/rclone
* pr feedback
* Dockerfile: specified reasonable defaults options for containerized kopia
* addressed pr comments, switched to gcr.io/distroless/static:nonroot
distroless has no executable code, so this requires KOPIA_PASSWORD
to always be provided via env, b/c distroless does not have
/bin/stty to disable TTY echo (we should not require that, BTW)
* site: added docker image documentation