Proxy-friendly version of install-docker-repository

[arnaudrichard]

Previously the script would not work in some proxy environments.
With this change, it is possible to have proxy set in "https_proxy" environment variable.

[gemini-code-assist]

Summary of Changes

Hello @arnaudrichard, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the install-docker-repository script by introducing full compatibility with environments that rely on an https_proxy. The core changes involve updating network requests to properly leverage proxy settings, thereby resolving previous issues where the script would fail to connect or download resources in such configurations.

Highlights

• Proxy Support: The script now properly utilizes the https_proxy environment variable for all network operations, enabling it to function correctly in proxy-configured environments.
• Network Reachability Check Improvement: The method for verifying download.docker.com reachability has been updated. It now uses curl instead of a raw TCP connection, making the check more robust, secure (using HTTPS), and proxy-aware.
• GPG Key Download Proxy Compatibility: The curl command used to fetch the Docker GPG key (docker.asc) has been modified to respect the https_proxy environment variable, ensuring successful key retrieval in proxy environments.
• Domain Name Correction: Minor text updates were made to consistently refer to download.docker.com instead of docker.download.com in print statements and comments.

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[gemini-code-assist]

Code Review

This pull request introduces proxy support for the Docker repository installation script, which is a valuable enhancement for users in corporate environments. The changes correctly use curl with proxy settings from the https_proxy environment variable. My review focuses on improving code quality with a few minor suggestions: fixing typos in user-facing messages, modernizing a shell command for better performance, and a recommendation to reduce code duplication for better maintainability.

[CLAassistant]

All committers have signed the CLA.