use an overlayfs on top of a read-only root filesystem

 This package adds functionality to an initramfs built by initramfs-tools.
 When installed and configured, the initramfs will mount an overlayfs
 filesystem on top of a read-only root volume.
 .
 The changes can be written to a in-memory temporary filesystem, a
 filesystem on an existing block device, or a dmcrypt encrypted block
 device.