Adds a multi-stage Dockerfile and scripts/netflow-db-docker.sh so new
users can run the pipeline with only Git and Docker installed, instead
of rustup, a C toolchain, and the autotools stack. The image clones the
nfdump fork at the superproject-pinned commit, so host submodules stay
uninitialized. Also adds gcc and git to shell.nix so the Nix shell fully
covers the native build, and documents the Docker path in docs/user.