docs: Add documentation for PRESET environment variable

- Add PRESET to the environment variables table
- Include detailed explanation of available preset values
- Add example showing how to use PRESET when generating a new map
- Document that PRESET is optional and only used with GENERATE_NEW_SAVE=true

Fixes #571

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

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

CLAUDE.md

@@ -0,0 +1,114 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This is a Docker image for running a Factorio headless server. It provides automated builds for multiple Factorio versions (stable and experimental) and supports both AMD64 and ARM64 architectures.
+
+## Architecture
+
+### Key Components
+
+1. **Docker Image Build System**
+   - `build.py` - Python script that builds Docker images from `buildinfo.json`
+   - `docker/Dockerfile` - Main Dockerfile that creates the Factorio server image
+   - `buildinfo.json` - Contains version info, SHA256 checksums, and tags for all supported versions
+   - Supports multi-architecture builds (linux/amd64, linux/arm64) using Docker buildx
+
+2. **Automated Updates**
+   - `update.sh` - Checks for new Factorio releases and updates `buildinfo.json`
+   - Updates README.md with new version tags
+   - Commits changes and tags releases automatically
+   - Run by GitHub Actions to keep images up-to-date
+
+3. **Container Scripts**
+   - `docker/files/docker-entrypoint.sh` - Main entrypoint that configures and starts the server
+   - `docker/files/docker-update-mods.sh` - Updates mods on server start
+   - `docker/files/docker-dlc.sh` - Manages DLC (Space Age) activation
+   - `docker/files/scenario.sh` - Alternative entrypoint for launching scenarios
+   - `docker/files/players-online.sh` - Checks if players are online (for watchtower integration)
+
+4. **RCON Client**
+   - `docker/rcon/` - C source for RCON client, built during Docker image creation
+   - Allows sending commands to the running server
+
+## Common Development Commands
+
+### Building Images
+
+```bash
+# Build a single architecture image locally
+python3 build.py
+
+# Build and push multi-architecture images
+python3 build.py --multiarch --push-tags
+```
+
+### Running the Container
+
+```bash
+# Basic run command
+docker run -d \
+  -p 34197:34197/udp \
+  -p 27015:27015/tcp \
+  -v /opt/factorio:/factorio \
+  --name factorio \
+  factoriotools/factorio
+
+# Using docker-compose
+docker-compose up -d
+```
+
+### Linting
+
+```bash
+# Lint Dockerfiles
+./lint.sh
+```
+
+### Testing Updates
+
+```bash
+# Check for new Factorio versions and update buildinfo.json
+./update.sh
+```
+
+## Key Configuration
+
+### Environment Variables
+- `LOAD_LATEST_SAVE` - Load the most recent save (default: true)
+- `GENERATE_NEW_SAVE` - Generate a new save if none exists (default: false)
+- `SAVE_NAME` - Name of the save file to load/create
+- `UPDATE_MODS_ON_START` - Update mods before starting (requires USERNAME/TOKEN)
+- `DLC_SPACE_AGE` - Enable/disable Space Age DLC (default: true)
+- `PORT` - UDP port for game server (default: 34197)
+- `RCON_PORT` - TCP port for RCON (default: 27015)
+
+### Volume Structure
+All data is stored in a single volume mounted at `/factorio`:
+```
+/factorio/
+├── config/           # Server configuration files
+├── mods/            # Game modifications
+├── saves/           # Save games
+├── scenarios/       # Scenario files
+└── script-output/   # Script output directory
+```
+
+## Version Management
+
+The project maintains compatibility with multiple Factorio versions:
+- Latest experimental version gets the `latest` tag
+- Latest stable version gets the `stable` tag
+- Each version also gets specific tags (e.g., `2.0.55`, `2.0`, `2`)
+- Legacy versions back to 0.12 are supported
+
+Version updates are automated via GitHub Actions that run `update.sh` periodically.
+
+## Testing Changes
+
+1. Modify `buildinfo.json` to test specific versions
+2. Run `python3 build.py` to build locally
+3. Test the container with your local data volume
+4. For production changes, ensure `update.sh` handles version transitions correctly

README.md

@@ -203,6 +203,22 @@ sudo docker run -d \
   factoriotools/factorio
 ```
 
+To generate a new map with a specific preset (e.g., death-world):
+
+```shell
+sudo docker run -d \
+  -p 34197:34197/udp \
+  -p 27015:27015/tcp \
+  -v /opt/factorio:/factorio \
+  -e LOAD_LATEST_SAVE=false \
+  -e GENERATE_NEW_SAVE=true \
+  -e SAVE_NAME=replaceme \
+  -e PRESET=death-world \
+  --name factorio \
+  --restart=unless-stopped \
+  factoriotools/factorio
+```
+
 ### Mods
 
 Copy mods into the mods folder and restart the server.
@@ -321,6 +337,7 @@ These are the environment variables which can be specified at container run time
 | BIND                 | IP address (v4 or v6) the server listens on (IP\[:PORT])             |                | 0.15+        |
 | RCON_PORT            | TCP port the rcon server listens on                                  | 27015          | 0.15+        |
 | SAVE_NAME            | Name to use for the save file                                        | _autosave1     | 0.17+        |
+| PRESET               | Map generation preset when GENERATE_NEW_SAVE is true                 |                | 0.17+        |
 | TOKEN                | factorio.com token                                                   |                | 0.17+        |
 | UPDATE_MODS_ON_START | If mods should be updated before starting the server                 |                | 0.17+        |
 | USERNAME             | factorio.com username                                                |                | 0.17+        |
@@ -330,6 +347,20 @@ These are the environment variables which can be specified at container run time
 
 **Note:** All environment variables are compared as strings
 
+#### PRESET Values
+
+The `PRESET` environment variable is used when generating a new map (when `GENERATE_NEW_SAVE=true`). It corresponds to Factorio's built-in map generation presets. Common values include:
+
+- `default` - Normal settings
+- `rich-resources` - Resources are more abundant
+- `marathon` - Recipes and technologies are more expensive
+- `death-world` - Biters are more aggressive and numerous
+- `death-world-marathon` - Combines death-world and marathon settings
+- `rail-world` - Resources are further apart, encouraging train usage
+- `ribbon-world` - Map height is limited for a unique challenge
+
+If PRESET is not specified or left empty, the map will be generated using the settings from `map-gen-settings.json` and `map-settings.json` without a preset.
+
 ## Container Details
 
 The philosophy is to [keep it simple](http://wiki.c2.com/?KeepItSimple).