MIL-013: Debian container and Windows Sandbox checks

A Debian image (python:3.13-slim-bookworm with Xvfb and GNOME's window
manager) runs the linters, the whole test suite, the screenshots and the
smoke test of a PyInstaller Linux binary and of the source program, so no
window opens on the development machine. The screenshot tool works on
Linux, a smoke driver presses the known button with the virtual mouse, and
Windows Sandbox scripts are written (not run, they open a window on the
host). README and the wiki Install pages describe it. Review records
RC-077 (Python) and RC-078 (shell).

Task: MIL-013#2
Task: MIL-013#3
Task: MIL-013#4
Task: MIL-013#5
Task: MIL-013#6
Task: MIL-013#7
Task: MIL-013#9
Task: MIL-013#10

tools/docker/screenshots.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Take the screenshots (level screen, card front, card back, window with the
+# menu bar) on the virtual desktop.
+#
+# Purpose:  screenshots without a desktop of the host (ADR-0005).
+# Usage:    screenshots.sh [output-folder]      (default /out)
+# Env:      see start-desktop.sh
+# Needs:    Pillow (the tools extra), the desktop
+# Exit:     0 when the four files were written; else the tool's exit code
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")/../.."
+# shellcheck source=tools/docker/start-desktop.sh
+. tools/docker/start-desktop.sh
+
+out="${1:-/out}"
+mkdir -p "${out}"
+python tools/capture_screenshots.py --output-dir "${out}"
+ls -l "${out}"