feat: add support for custom go-download-site input

[abdennour]

This adds a new go-download-site input parameter that allows users to specify a custom base URL for downloading Go distributions. This is particularly useful for GitHub Enterprise Server (GHES) environments, corporate proxies, or air-gapped setups where direct access to github.com is restricted.

Changes:

• Added go-download-site input to action.yml (default: https://github.com)
• Updated installer.ts to replace the default GitHub URL with the provided custom site URL
• Updated main.ts to pass the input to the installer
• Added documentation in README.md with examples for Artifactory and GHES proxies

Resolves #296

Description:
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Related issue:
Add link to the related issue.

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull request overview

This PR adds support for custom Go distribution download sites through a new go-download-site input parameter. This feature enables users in restricted network environments (GitHub Enterprise Server, corporate proxies, air-gapped setups) to specify alternative base URLs for downloading Go distributions from mirrored repositories.

Key Changes:

• Adds go-download-site input parameter with default value https://github.com
• Implements URL replacement logic in the installer to use custom download sites for manifest-based distributions
• Provides comprehensive documentation with examples for Artifactory and GHES proxy configurations

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
action.yml	Adds the new go-download-site input parameter with default value and description
src/main.ts	Retrieves the go-download-site input and passes it to the installer
src/installer.ts	Threads the goDownloadSite parameter through the call chain and implements URL replacement in getInfoFromManifest()
README.md	Adds comprehensive documentation section explaining the feature, use cases, requirements, and examples

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[Copilot]

The example URL https://internal-artifactory.company.com/github-proxy should clarify whether it should end with a trailing slash or not. The implementation uses simple string replacement, so the URL format must exactly match how GitHub structures its URLs. Consider adding a note:

"Important: The custom download site URL should not include a trailing slash, as it directly replaces https://github.com in the full download URL path."

Suggested change

	**Important:** The custom download site URL should not include a trailing slash, as it directly replaces `https://github.com` in the full download URL path.

[Copilot]

The URL replacement has two potential issues:

1. String replace is not precise: Using replace() with a string will replace any occurrence of 'https://github.com' in the URL, not just at the beginning. Use a regex with ^ anchor for safety.

2. No trailing slash handling: If goDownloadSite ends with a trailing slash, the result will have a double slash (e.g., https://example.com//actions/...).

Suggested fix:

const normalizedSite = goDownloadSite.replace(/\/$/, '');
info.downloadUrl = rel.files[0].download_url.replace(/^https:\/\/github\.com/, normalizedSite);

This ensures only the base URL is replaced and handles trailing slashes gracefully.

Suggested change

	info.downloadUrl = rel.files[0].download_url.replace('https://github.com', goDownloadSite);
	const normalizedSite = goDownloadSite.replace(/\/$/, '');
	info.downloadUrl = rel.files[0].download_url.replace(/^https:\/\/github\.com/, normalizedSite);

[Copilot]

This note indicates that the go-download-site parameter doesn't affect fallback downloads from go.dev, but the implementation in installer.ts only modifies manifest-based downloads. Consider clarifying that:

1. The parameter only affects downloads from the go-versions repository manifest
2. When manifest downloads fail, the action falls back to go.dev without using the custom download site
3. This limitation means truly air-gapped environments may still encounter issues if manifest downloads aren't available

This could be made clearer to help users understand when they might still need additional configuration for fully restricted environments.

Suggested change

> **Note:** The `go-download-site` parameter only affects downloads from the go-versions repository. Direct downloads from go.dev (fallback mechanism) are not affected by this setting.

> **Note:** The `go-download-site` parameter only affects downloads from the go-versions repository manifest. If manifest-based downloads fail, the action falls back to downloading directly from go.dev, and the custom download site is not used. This means that in truly air-gapped environments, you may still encounter issues if manifest downloads aren't available—additional configuration or pre-caching is required for fully restricted environments.

[Copilot]

[nitpick] Consider adding a note about the manifest URL requirement. The action still fetches the versions manifest from https://raw.githubusercontent.com/actions/go-versions/main/versions-manifest.json (line 17 in installer.ts), which may not be accessible in restricted environments. Users may need to configure their proxy/mirror to also handle this manifest URL or ensure it's accessible through their network configuration.

Suggested change

	3. Be accessible from your runners
	3. Be accessible from your runners
	4. Ensure that the versions manifest file (`https://raw.githubusercontent.com/actions/go-versions/main/versions-manifest.json`) is accessible from your runners.
	- **Note:** The action always fetches the manifest from this public URL, even when using a custom download site. In restricted environments, you may need to mirror or proxy this manifest file, or configure your network to allow access.

[Copilot]

[nitpick] The fallback || 'https://github.com' is redundant since go-download-site has a default value in action.yml. The core.getInput() will return the default value from action.yml if not explicitly provided by the user. While this defensive approach doesn't cause issues, it could be simplified to just:

const goDownloadSite = core.getInput('go-download-site');

This makes the code cleaner and relies on the single source of truth for the default value in action.yml.

Suggested change

	const goDownloadSite = core.getInput('go-download-site') || 'https://github.com';
	const goDownloadSite = core.getInput('go-download-site');