feat: Add rootless Docker support (#574)

* feat: Add rootless Docker support

Implements #547 - Add support for rootless Docker images to avoid permission issues.

Key changes:
- Add Dockerfile.rootless that runs as UID 1000 by default
- Create simplified entrypoint script without chown operations
- Add build-rootless.py to build rootless variants with -rootless suffix
- Document rootless usage in README-ROOTLESS.md
- Update main README with rootless section

The rootless images eliminate common permission problems by:
- Running as non-root from the start (USER 1000:1000)
- Avoiding recursive chown operations that can cause race conditions
- Using open permissions (777) on directories during build
- Not supporting PUID/PGID environment variables

This provides a cleaner solution for rootless Docker users and those
experiencing permission issues with volumes.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fix: Address linting issues in rootless Docker implementation

- Add --no-install-recommends to apt-get install in Dockerfile
- Consolidate consecutive RUN instructions in Dockerfile
- Fix shellcheck warnings: quote variables and use -n instead of \! -z
- These changes improve best practices without affecting functionality

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* feat: Add rootless image building to CI pipeline

- Update docker-build.yml workflow to build rootless variants
- Rootless images are built after regular images with -rootless suffix
- Both use the same multi-architecture build process
- Triggered automatically when buildinfo.json changes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* refactor: Unify build system for regular and rootless images

- Create build-unified.py that handles both regular and rootless builds
- Convert build.py and build-rootless.py to wrapper scripts for backwards compatibility
- Update CI workflow to use unified build command
- Add BUILD_MIGRATION.md documentation
- Eliminate code duplication between build scripts
- Support flexible build options: --rootless, --both, --only-stable-latest

This maintains all existing functionality while providing a cleaner, more maintainable build system.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* chore: Add Python cache to .gitignore and remove from repo

- Add __pycache__/ and Python compiled files to .gitignore
- Remove accidentally committed __pycache__ directory
- Prevent future Python cache files from being tracked

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* refactor: Replace build system with unified solution

- Remove old build.py and build-rootless.py wrapper scripts
- Rename build-unified.py to build.py as the main build script
- Delete BUILD_MIGRATION.md (no longer needed)
- Update CI workflow to use new build.py syntax
- Update documentation in CLAUDE.md and README-ROOTLESS.md

The new build system provides all functionality in a single script:
- Default: builds regular images
- --rootless: builds only rootless images
- --both: builds both regular and rootless images
- --multiarch and --push-tags: work as before

This creates a cleaner, more maintainable build system.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* docs: Consolidate rootless documentation and mark as experimental

- Remove separate README-ROOTLESS.md file
- Integrate rootless documentation into main README.md
- Mark rootless support as experimental
- Add clear documentation about limitations and use cases
- Include warning about experimental nature

This consolidates all documentation in one place and makes it clear
that rootless support is still experimental.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -11,8 +11,9 @@ This is a Docker image for running a Factorio headless server. It provides autom
 ### Key Components
 
 1. **Docker Image Build System**
-   - `build.py` - Python script that builds Docker images from `buildinfo.json`
+   - `build.py` - Unified Python script that builds both regular and rootless Docker images from `buildinfo.json`
    - `docker/Dockerfile` - Main Dockerfile that creates the Factorio server image
+   - `docker/Dockerfile.rootless` - Dockerfile for rootless variant (runs as UID 1000)
    - `buildinfo.json` - Contains version info, SHA256 checksums, and tags for all supported versions
    - Supports multi-architecture builds (linux/amd64, linux/arm64) using Docker buildx
 
@@ -38,11 +39,20 @@ This is a Docker image for running a Factorio headless server. It provides autom
 ### Building Images
 
 ```bash
-# Build a single architecture image locally
+# Build regular images locally (single architecture)
 python3 build.py
 
-# Build and push multi-architecture images
+# Build rootless images only
+python3 build.py --rootless
+
+# Build both regular and rootless images
+python3 build.py --both
+
+# Build and push multi-architecture images (regular only)
 python3 build.py --multiarch --push-tags
+
+# Build and push both regular and rootless multi-architecture images
+python3 build.py --multiarch --push-tags --both
 ```
 
 ### Running the Container
@@ -109,6 +119,8 @@ Version updates are automated via GitHub Actions that run `update.sh` periodical
 ## Testing Changes
 
 1. Modify `buildinfo.json` to test specific versions
-2. Run `python3 build.py` to build locally
+2. Run `python3 build.py` to build regular images locally
+   - Use `python3 build.py --rootless` for rootless images
+   - Use `python3 build.py --both` to build both variants
 3. Test the container with your local data volume
 4. For production changes, ensure `update.sh` handles version transitions correctly