Windows-Containers Add Docker update functionality with configuration preservation to install-docker-ce.ps1

This PR enhances the install-docker-ce.ps1 script to support updating existing Docker installations while preserving the daemon.json configuration file, eliminating the need for manual backup/restore steps.

Problem

Previously, updating Docker required a complex 7-step manual process:

Stop Docker service
Remove Docker service
Backup C:\ProgramData\docker\config\daemon.json
Run install script
Stop Docker service again
Restore daemon.json configuration
Start Docker service

The script would either skip installation entirely if Docker was already installed, or perform a clean install that overwrote existing configuration.

Solution

Added a new -Update switch parameter that automates the entire update process:

# Update existing Docker installation while preserving config
.\install-docker-ce.ps1 -Update

# Update with specific version
.\install-docker-ce.ps1 -Update -DockerVersion "20.10.17"

Key Features

Automated Update Process:

Validates and backs up existing daemon.json configuration
Gracefully stops and unregisters Docker service
Installs new Docker binaries
Registers service with preserved configuration
Handles cleanup automatically

Robust Error Handling:

Validates JSON content before backup
Graceful handling of service operation failures
Automatic config restoration if update fails
Comprehensive error messages and warnings

Backwards Compatibility:

All existing functionality preserved unchanged
New parameter is optional with no impact on existing scripts
Default behavior remains identical

Technical Implementation

New Functions: Backup-DockerConfig() and Restore-DockerConfig() with validation
Enhanced Logic: Modified Install-ContainerHost() and Install-Docker() to handle update scenarios
Error Recovery: Automatic restoration of configuration if update fails partway through

Testing

The implementation includes comprehensive error handling for edge cases:

Missing or corrupted daemon.json files
Permission issues during backup/restore operations
Docker service operation failures
Failed updates with automatic recovery

Fixes #595.

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