Hosts running Docker with the default Debian/Ubuntu iptables use the
nf_tables backend (iptables-nft). Inserting rules via iptables-legacy
created them in a separate, unreferenced table — bans were recorded in
fail2ban but packets were never dropped.
Switching action commands to iptables-nft writes into the same
DOCKER-USER chain that Docker manages, so bans take effect immediately.
Also reverts the update-alternatives override from the Dockerfile since
it is no longer needed (and generated noisy warnings).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>