Add essential initramfs integration for composefs support

- Add initramfs integration files based on debian-bootc project
- Include bootc-initramfs-setup.service for systemd integration
- Add dracut module-setup.sh for initramfs generation
- Include prepare-root.conf for OSTree composefs configuration
- Update Containerfile examples to include initramfs files
- Fix systemctl calls to use symlinks instead of systemctl commands
- Add comprehensive initramfs-integration.md documentation
- Update README to reference new initramfs documentation

Based on files from https://github.com/bootcrew/debian-bootc:
- bootc-initramfs-setup.service
- module-setup.sh
- prepare-root.conf

These files are essential for proper composefs support and boot
functionality in Debian bootc images.

initramfs-integration.md

@@ -0,0 +1,178 @@
+# Initramfs Integration for Debian bootc Images
+
+This document explains the essential initramfs integration files required for proper composefs support and boot functionality in Debian bootc images.
+
+## Overview
+
+The initramfs integration provides:
+- **Composefs Support**: Enables composefs mounting during early boot
+- **Systemd Integration**: Proper systemd service management in initramfs
+- **Dracut Module**: Automatic inclusion of bootc setup in initramfs
+- **Configuration**: OSTree and composefs configuration
+
+## Required Files
+
+### 1. Systemd Service: `bootc-initramfs-setup.service`
+
+**Location**: `/usr/lib/systemd/system/bootc-initramfs-setup.service`
+
+```ini
+[Unit]
+DefaultDependencies=no
+ConditionKernelCommandLine=composefs
+ConditionPathExists=/etc/initrd-release
+After=sysroot.mount
+Requires=sysroot.mount
+Before=initrd-root-fs.target
+Before=initrd-switch-root.target
+OnFailure=emergency.target
+OnFailureJobMode=isolate
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/bootc-initramfs-setup
+StandardInput=null
+StandardOutput=journal
+StandardError=journal+console
+RemainAfterExit=yes
+```
+
+**Purpose**: 
+- Runs during initramfs boot when `composefs` is in kernel command line
+- Executes the bootc setup script before root filesystem switch
+- Handles composefs-specific initialization
+
+### 2. Dracut Module: `module-setup.sh`
+
+**Location**: `/usr/lib/dracut/modules.d/37composefs/module-setup.sh`
+
+```bash
+#!/usr/bin/bash
+check() {
+    return 0
+}
+
+depends() {
+    return 0
+}
+
+install() {
+    inst \
+        "${moddir}/bootc-initramfs-setup" /bin/bootc-initramfs-setup
+    inst \
+        "${moddir}/bootc-initramfs-setup.service" \
+        "${systemdsystemunitdir}/bootc-initramfs-setup.service"
+    $SYSTEMCTL -q --root "${initdir}" add-wants \
+        'initrd-root-fs.target' 'bootc-initramfs-setup.service'
+}
+```
+
+**Purpose**:
+- Dracut module that installs bootc components into initramfs
+- Copies the setup script and systemd service
+- Adds the service as a dependency of `initrd-root-fs.target`
+
+### 3. OSTree Configuration: `prepare-root.conf`
+
+**Location**: `/usr/lib/ostree-boot/prepare-root.conf`
+
+```ini
+[composefs]
+enabled = yes
+
+[sysroot]
+readonly = true
+```
+
+**Purpose**:
+- Enables composefs support in OSTree
+- Sets sysroot to readonly mode for security
+- Configures OSTree to use composefs for root filesystem
+
+## Integration in Containerfile
+
+Add these files to your base image build:
+
+```dockerfile
+# Copy initramfs integration files
+COPY files/37composefs/bootc-initramfs-setup.service /usr/lib/systemd/system/
+COPY files/37composefs/module-setup.sh /usr/lib/dracut/modules.d/37composefs/
+COPY files/ostree/prepare-root.conf /usr/lib/ostree-boot/prepare-root.conf
+
+# Make module-setup.sh executable
+RUN chmod +x /usr/lib/dracut/modules.d/37composefs/module-setup.sh
+```
+
+## Directory Structure
+
+Create this directory structure in your build context:
+
+```
+files/
+├── 37composefs/
+│   ├── bootc-initramfs-setup.service
+│   └── module-setup.sh
+└── ostree/
+    └── prepare-root.conf
+```
+
+## Kernel Command Line Requirements
+
+For composefs support, ensure your kernel command line includes:
+
+```
+composefs=sha256:...
+```
+
+The composefs parameter should point to the composefs image containing your root filesystem.
+
+## Dependencies
+
+These files require:
+
+- **systemd**: For service management
+- **dracut**: For initramfs generation
+- **ostree**: For composefs support
+- **bootc-initramfs-setup**: The actual setup script (provided by bootc)
+
+## Testing
+
+To test initramfs integration:
+
+1. **Build the image** with initramfs files included
+2. **Generate initramfs** using dracut
+3. **Boot the system** with composefs kernel parameter
+4. **Verify** that composefs mounting works correctly
+
+## Troubleshooting
+
+### Common Issues
+
+1. **Service not starting**: Check that `composefs` is in kernel command line
+2. **Module not found**: Ensure dracut module is in correct location
+3. **Permission denied**: Make sure `module-setup.sh` is executable
+4. **Composefs not enabled**: Verify `prepare-root.conf` is in correct location
+
+### Debug Commands
+
+```bash
+# Check if service is enabled
+systemctl list-unit-files | grep bootc-initramfs-setup
+
+# Verify dracut module
+ls -la /usr/lib/dracut/modules.d/37composefs/
+
+# Check composefs configuration
+cat /usr/lib/ostree-boot/prepare-root.conf
+
+# Test initramfs generation
+dracut --add composefs --force
+```
+
+## Source
+
+These files are based on the [debian-bootc project](https://github.com/bootcrew/debian-bootc) which provides Debian-specific bootc integration.
+
+## License
+
+The files are licensed under the GNU Lesser General Public License v2.1 or later, as indicated in the copyright header.