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/target.Dockerfile

@@ -0,0 +1,16 @@
+# SSH target for the netssh test environment: every NetBox host, the bastion,
+# and the static host run this image with a different hostname.
+FROM alpine:3.23
+
+# Alpine locks new accounts with "!"; sshd refuses key logins for locked
+# accounts, so the password field is set to "*" (no password, not locked).
+RUN apk add --no-cache openssh-server openssh-sftp-server python3 \
+ && for user in ops admin; do \
+      adduser -D -s /bin/sh "$user" && sed -i "s/^$user:!/$user:*/" /etc/shadow; \
+    done
+
+COPY testenv/targets/sshd_config /etc/ssh/sshd_config
+COPY testenv/targets/entrypoint.sh /entrypoint.sh
+
+EXPOSE 22 2222
+ENTRYPOINT ["/entrypoint.sh"]