Modernize Docker installer and add CI tests

[foxly-it]

Summary

• modernize the Docker APT setup to use the current Deb822 docker.sources format and docker.asc signing key
• harden CLI parsing, OS validation, package handling, backups, installation verification, and explicit data-removal behavior
• expand the German and English documentation with current support, usage, safety, backup, and logging details
• add isolated integration simulations and a GitHub Actions workflow with syntax checks and ShellCheck

Why

The previous installer contained stale repository handling, incomplete package lists, ineffective options, unsafe non-interactive data-removal behavior, and documentation that no longer matched the implementation. The update makes destructive behavior explicit and aligns the installer with Docker's current Debian and Ubuntu instructions.

Impact

Users get a safer installer with predictable automation semantics, current repository configuration, clearer logs, and post-install verification of Docker Engine, Compose, Buildx, daemon access, and optional hello-world execution.

Validation

• bash -n dockerinstall.sh tests/integration.sh tests/mock-command.sh
• bash tests/integration.sh
• git diff --check origin/main...HEAD
• simulated install flows for Debian 13 and Ubuntu 24.04
• supported-release self-check matrix for Debian 11/12/13 and Ubuntu 22.04/24.04/25.10/26.04
• upgrade with backup and forced reinstall
• uninstall with retained and explicitly removed data