Add custom executable support for Docker-based Prettier workflows

[Copilot]

Description

Enables running Prettier through arbitrary executables (Docker, wrapper scripts) via new prettier.customExecutable setting. Addresses Docker-centric development where Prettier runs inside containers.

Implementation:

• PrettierExecutableInstance: New class implementing PrettierInstance interface. Executes Prettier via spawn with stdin/stdout, supports all CLI operations (format, file-info, support-info, config resolution). Single-quote shell escaping on all dynamic arguments.

• ModuleResolverNode: Added getCustomExecutableInstance method. Custom executable checked first in resolution chain, instance caching maintained.

• Configuration: prettier.customExecutable setting in package.json. Optional ${prettier} placeholder for Prettier path. Restricted in untrusted workspaces.

• Localization: English, Chinese Simplified, Chinese Traditional strings added.

Usage:

// Docker
{
  "prettier.customExecutable": "docker compose exec -T app node_modules/.bin/prettier"
}

// With placeholder
{
  "prettier.customExecutable": "docker exec container ${prettier}",
  "prettier.prettierPath": "/app/node_modules/.bin/prettier"
}

Security:

• All dynamic arguments (paths, options) escaped via single-quote method
• CodeQL scan passes with 0 alerts
• Custom executable itself is trusted user configuration

Test fixture in test-fixtures/custom-executable/ with wrapper script example.

Related Issue

Fixes #232

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/Build changes

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the code style of this project (npm run lint passes)
• I have run npm run prettier to format my code
• I have added/updated tests that prove my fix or feature works
• All new and existing tests pass (npm test)
• I have updated the CHANGELOG.md with a summary of my changes

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com//advisories
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Custom Prettier Resolution v2</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.
In #232, prettier-vscode added support for a prettier.prettierPath configuration. This works great for things like monorepos where the node_modules/ directory (and thus the Prettier binary) might lie outside the current workspace.

This does not, however, solve the problem for Docker-centric workspaces. We use Docker extensively in development so developers have an easier time getting a standardized development environment running. Local files (e.g. ./app/, ./src/, etc.) are mounted into a running Docker container via docker-compose. This container has all the development dependencies needed, including specific versions of Unix, node, Ruby, etc.

Everything is configured so that this works from the command line:

$ docker compose exec app node_modules/.bin/prettier './**/*' --ignore-unknown

I tried to set prettier.prettierPath: "docker compose exec app node_modules/.bin/prettier", but that failed.

Describe the solution you'd like
I'd like to be able to specify an arbitrary executable (here docker) with arguments (here compose exec app node_modules/.bin/prettier") for VSCode-Prettier to run.

Describe alternatives you've considered
I've successfully used emeraldwalk.runonsave to solve this, but it doesn't have the Prettier-specific niceties.

Additional context
Possible downsides:

• Use with Docker requires passing workspace-relative paths to the executable.
• Use with Docker requires mounting files. That is outside the scope of Prettier-VSCode. If Docker containers have particularly complex configurations, project maintainers may have to write a wrapper around Prettier to munge paths or arguments.

Alternative solution: use a Visual Studio Code Dev Container to develop inside the Docker container, then run prettier-vscode normally.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Custom Prettier Resolution v2 #2820

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