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.Dockerfile

@@ -0,0 +1,23 @@
+# Client for the netssh test environment: netssh built from this repository
+# plus everything it drives or integrates with (ssh, sftp, scp, tmux, zellij,
+# bash, zsh, fish). Rebuild after code changes: testenv/up.sh or
+# docker compose -f testenv/compose.yaml build client
+FROM golang:1.27.4-alpine3.23 AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY cmd ./cmd
+COPY internal ./internal
+RUN CGO_ENABLED=0 go build -o /out/netssh ./cmd/netssh
+
+FROM alpine:3.23
+RUN apk add --no-cache openssh-client-default tmux zellij bash zsh fish bash-completion \
+      curl python3 jq ncurses coreutils \
+ && adduser -D -s /bin/bash tester
+COPY --from=build /out/netssh /usr/local/bin/netssh
+COPY testenv/client-entrypoint.sh /entrypoint.sh
+USER tester
+WORKDIR /home/tester
+ENV TERM=xterm-256color LANG=C.UTF-8
+ENTRYPOINT ["/entrypoint.sh"]
+CMD ["sleep", "infinity"]