Create containers/nettest/default.nix using dockerTools.buildLayeredImage
with the same tools as the Dockerfile (curl, jq, dnsutils, cacert, bash).
Update container-list and container-tag-and-release to handle containers
that have both a Dockerfile and default.nix, requiring --nix or --dockerfile
flag when both exist.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>