test: add a local Docker test environment for all features

testenv/ starts two real NetBox 4.7 instances (v2 and v1 token), SSH
targets on public, mgmt, and prod networks, a jump host, an IPv6-only
host, a static host, and a client with ssh, sftp, scp, tmux, zellij,
bash, zsh, and fish. A seed script fills NetBox through the REST API.

141 end-to-end checks cover cache and refresh filters, connecting,
all resolver strategies, multiple instances, shortcuts and rewrites,
SSH options and rules, static hosts, sftp/scp incl. IPv6 and jump
hosts, tunnels, local notes and NetBox comments, completion and shell
hooks, the TUI (driven through tmux), the configurator, and uninstall.

Local only: testenv/up.sh, test.sh, shell.sh, down.sh. Not part of
CI/CD.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

testenv/targets/entrypoint.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+# Prepares host keys and the test users, starts a small HTTP server for the
+# tunnel tests (reachable only on the target itself), then runs sshd.
+set -eu
+
+[ -f /etc/ssh/ssh_host_ed25519_key ] || ssh-keygen -q -t ed25519 -N '' -f /etc/ssh/ssh_host_ed25519_key
+
+for user in ops admin; do
+  home="/home/$user"
+  mkdir -p "$home/.ssh"
+  cp /keys/authorized_keys "$home/.ssh/authorized_keys"
+  chown -R "$user:$user" "$home/.ssh"
+  chmod 700 "$home/.ssh"
+  chmod 600 "$home/.ssh/authorized_keys"
+done
+
+mkdir -p /srv/www
+echo "hello from $(hostname)" > /srv/www/index.html
+python3 -m http.server 8081 --bind 127.0.0.1 --directory /srv/www >/dev/null 2>&1 &
+
+exec /usr/sbin/sshd -D -e