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/client-entrypoint.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# ssh reads ~/.ssh from the passwd entry, not $HOME, so the key and ssh config
+# live in /home/tester for every test; tests isolate netssh via HOME/XDG_*.
+set -eu
+mkdir -p /home/tester/.ssh
+if [ -f /keys/id_ed25519 ]; then
+  install -m 600 /keys/id_ed25519 /home/tester/.ssh/id_ed25519
+fi
+cat >/home/tester/.ssh/config <<'EOF'
+Host *
+  IdentityFile ~/.ssh/id_ed25519
+  IdentitiesOnly yes
+  BatchMode yes
+  ConnectTimeout 5
+  StrictHostKeyChecking accept-new
+  LogLevel ERROR
+EOF
+chmod 700 /home/tester/.ssh
+chmod 600 /home/tester/.ssh/config
+exec "$@"