Add support for multiple Docker registries including ghcr.io

This commit adds comprehensive multi-registry support to the Kamal
Accessories Updater, starting with GitHub Container Registry (ghcr.io).

Key Changes:
- Added parse_image_name() function to parse registry prefix from image names
  (e.g., ghcr.io/org/image, docker.io/library/redis)
- Added detect_registry_type() function to identify registry type
- Implemented GHCR-specific functions:
  * get_latest_version_ghcr() - Fetches tags from GHCR with token auth
  * get_image_sha256_ghcr() - Retrieves SHA256 digests from GHCR
- Refactored Docker Hub functions into registry-specific implementations:
  * get_latest_version_dockerhub()
  * get_image_sha256_dockerhub()
- Updated main dispatcher functions to route to appropriate registry handlers
- Enhanced caching to support multiple registries with separate cache keys
- Added GHCR_TOKEN environment variable support for authentication

Registry Support:
- Docker Hub (docker.io) - Full support, no auth required for public images
- GitHub Container Registry (ghcr.io) - Full support with token authentication
- Google Container Registry (gcr.io) - Detection ready
- Quay.io - Detection ready
- Generic OCI registries - Basic detection

Documentation:
- Updated README with multi-registry support information
- Added "Supported Registries" section with examples
- Documented GHCR authentication requirements
- Added configuration examples for different registries
- Updated "How It Works" section to reflect registry detection

The implementation maintains backward compatibility with existing Docker Hub
configurations while enabling users to specify registry prefixes for images
from other registries.

Author: claude

README.md

@@ -3,11 +3,12 @@
 [![Test](https://github.com/robfrank/kamal-accessories-updater/actions/workflows/test.yml/badge.svg)](https://github.com/robfrank/kamal-accessories-updater/actions/workflows/test.yml)
 [![Release](https://github.com/robfrank/kamal-accessories-updater/actions/workflows/release.yml/badge.svg)](https://github.com/robfrank/kamal-accessories-updater/actions/workflows/release.yml)
 
-A GitHub Action that automatically checks for and updates [Kamal](https://kamal-deploy.org/) accessories to their latest versions from Docker Hub.
+A GitHub Action that automatically checks for and updates [Kamal](https://kamal-deploy.org/) accessories to their latest versions from Docker Hub, GitHub Container Registry (GHCR), and other OCI registries.
 
 ## Features
 
-- 🔍 **Automatic Version Detection** - Scans your Kamal deployment configurations for accessories and checks Docker Hub for latest versions
+- 🔍 **Automatic Version Detection** - Scans your Kamal deployment configurations for accessories and checks registries for latest versions
+- 🌐 **Multi-Registry Support** - Works with Docker Hub, GitHub Container Registry (ghcr.io), and other OCI-compliant registries
 - 📦 **Semantic Versioning** - Intelligently compares semantic versions to ensure only newer versions are applied
 - 🔒 **SHA256 Support** - Automatically fetches and includes SHA256 digests for enhanced security
 - 📝 **Pull Request Creation** - Optionally creates pull requests with detailed update information
@@ -139,11 +140,12 @@ Check for updates and create a PR:
 
 1. **Scans Configuration Files** - Finds all `deploy*.yml` files in your config directory
 2. **Extracts Accessories** - Parses YAML to identify accessories and their current versions
-3. **Checks Docker Hub** - Queries Docker Hub API for latest semantic versions
-4. **Compares Versions** - Intelligently compares versions to determine if updates are available
-5. **Fetches Digests** - Retrieves SHA256 digests for the latest versions
-6. **Updates Files** - Modifies your configuration files with new versions and digests
-7. **Creates PR** - Optionally creates a pull request with the changes
+3. **Detects Registry** - Automatically detects the registry (Docker Hub, GHCR, etc.) from the image name
+4. **Queries Registry API** - Fetches latest semantic versions from the appropriate registry
+5. **Compares Versions** - Intelligently compares versions to determine if updates are available
+6. **Fetches Digests** - Retrieves SHA256 digests for the latest versions
+7. **Updates Files** - Modifies your configuration files with new versions and digests
+8. **Creates PR** - Optionally creates a pull request with the changes
 
 ## Supported Accessories
 
@@ -156,37 +158,112 @@ This action works with any Docker image used as a Kamal accessory. Common exampl
 - BusyBox (`busybox`)
 - Any custom Docker image on Docker Hub
 
+## Supported Registries
+
+The action supports multiple container registries:
+
+### Docker Hub (docker.io)
+
+**Default registry** - No authentication required for public images.
+
+```yaml
+accessories:
+  redis:
+    image: redis:7.0.0                    # Official image (implicit docker.io)
+
+  custom:
+    image: myorg/myapp:1.0.0             # Organization image
+
+  explicit:
+    image: docker.io/library/postgres:15 # Explicit registry prefix
+```
+
+### GitHub Container Registry (ghcr.io)
+
+**Requires authentication** for most operations. Set the `GHCR_TOKEN` environment variable with a GitHub Personal Access Token (PAT) or use the automatic `GITHUB_TOKEN`.
+
+```yaml
+accessories:
+  myapp:
+    image: ghcr.io/myorg/myapp:1.0.0
+
+  public:
+    image: ghcr.io/owner/public-image:2.0.0
+```
+
+**GitHub Action Configuration for GHCR:**
+
+```yaml
+- name: Update accessories
+  uses: robfrank/kamal-accessories-updater@v1
+  with:
+    config-dir: config
+  env:
+    GHCR_TOKEN: ${{ secrets.GITHUB_TOKEN }}  # Use GitHub token for GHCR access
+```
+
+**For private GHCR images**, create a GitHub PAT with `read:packages` scope:
+
+```yaml
+env:
+  GHCR_TOKEN: ${{ secrets.GHCR_PAT }}  # Use custom PAT for private images
+```
+
+### Other Registries
+
+The action includes detection for:
+- **Google Container Registry** (gcr.io)
+- **Quay.io** (quay.io)
+- **Generic OCI registries** (basic support)
+
+Support for additional registries is being actively developed. Open an issue if you need support for a specific registry.
+
 ## Configuration File Format
 
 Your Kamal configuration files should follow the standard format:
 
 ```yaml
 accessories:
+  # Docker Hub - official image (default registry)
   redis:
     image: redis:7.0.0
     host: 192.168.0.1
     # ... other configuration
 
+  # Docker Hub - with SHA256 digest
   postgres:
     image: postgres:15.0@sha256:abc123...
     host: 192.168.0.1
     # ... other configuration
+
+  # GitHub Container Registry
+  myapp:
+    image: ghcr.io/myorg/myapp:1.2.3@sha256:def456...
+    host: 192.168.0.1
+    # ... other configuration
+
+  # Docker Hub - organization image
+  custom:
+    image: mycompany/myservice:2.0.0
+    host: 192.168.0.1
+    # ... other configuration
 ```
 
 The action will:
+- Automatically detect the registry from the image name
 - Preserve your existing configuration structure
 - Update only the image version
 - Add or update SHA256 digests
 - Keep all other settings intact
 
 ## Caching
 
-The action caches Docker Hub API responses for 1 hour to:
+The action caches registry API responses for 1 hour to:
 - Reduce API calls
 - Improve performance
 - Avoid rate limiting
 
-Cache is stored in `/tmp/docker-registry-cache` and automatically cleaned up.
+Cache is stored in `/tmp/docker-registry-cache` and automatically cleaned up. Each registry is cached separately to ensure accuracy.
 
 ## Testing
 
@@ -256,6 +333,8 @@ This project is licensed under the MIT License - see the [LICENSE](LICENSE) file
 - [Kamal Documentation](https://kamal-deploy.org/)
 - [Kamal Accessories Guide](https://kamal-deploy.org/docs/configuration/accessories/)
 - [Docker Hub API](https://docs.docker.com/registry/spec/api/)
+- [GitHub Container Registry](https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry)
+- [OCI Distribution Spec](https://github.com/opencontainers/distribution-spec)
 
 ## Acknowledgments