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.

building/base-images.md

@@ -36,6 +36,67 @@ make
 sudo make install
 ```
 
+## Essential Initramfs Integration
+
+**⚠️ CRITICAL**: For proper composefs support and boot functionality, you must include initramfs integration files in your base image. These files are essential for Debian bootc images and are based on the [debian-bootc project](https://github.com/bootcrew/debian-bootc).
+
+### Required Files
+
+Create these files in your build context:
+
+**1. `files/37composefs/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
+```
+
+**2. `files/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'
+}
+```
+
+**3. `files/ostree/prepare-root.conf`**:
+```ini
+[composefs]
+enabled = yes
+
+[sysroot]
+readonly = true
+```
+
 ### Step 1: Create Containerfile
 
 Create a `Containerfile.base`:
@@ -51,12 +112,18 @@ RUN apt update && apt install -y \
     systemd-tmpfiles \
     kernel \
     initramfs-tools \
+    dracut \
     grub2 \
     grub2-common \
     efibootmgr \
     ostree \
     && apt clean
 
+# 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
+
 # Create essential directories
 RUN mkdir -p /usr/lib/systemd/system \
     /usr/lib/systemd/user \
@@ -69,12 +136,15 @@ RUN mkdir -p /usr/lib/systemd/system \
 RUN ln -sf /lib/systemd/systemd /sbin/init
 
 # Set up basic systemd configuration
-RUN systemctl set-default multi-user.target
+RUN ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
 
 # Create essential systemd services
-RUN systemctl enable systemd-resolved.service \
-    systemd-networkd.service \
-    systemd-timesyncd.service
+RUN ln -sf /usr/lib/systemd/system/systemd-resolved.service \
+    /etc/systemd/system/multi-user.target.wants/systemd-resolved.service
+RUN ln -sf /usr/lib/systemd/system/systemd-networkd.service \
+    /etc/systemd/system/multi-user.target.wants/systemd-networkd.service
+RUN ln -sf /usr/lib/systemd/system/systemd-timesyncd.service \
+    /etc/systemd/system/multi-user.target.wants/systemd-timesyncd.service
 
 # Configure basic networking
 RUN echo -e "[Match]\nName=*\n\n[Network]\nDHCP=yes" > /etc/systemd/network/80-dhcp.network