Init Layer in Docker

Question

Why is there a containerID-init layer created when a container is created from an image in Docker? Couldn't the new container have as parent the image?

Answer 1

Each containers has two layers, one (called the init layer), which is based on an image layer and a child of that which contains the actual container content. The init layer contains a few files that must always exist in Docker containers (e.g. /.dockerinit). Supported Filesystems

The containerID-init layer is the init layer of a container which is based on an image. It add some file into current container,include:

    "/dev/pts":         "dir",
    "/dev/shm":         "dir",
    "/proc":            "dir",
    "/sys":             "dir",
    "/.dockerenv":      "file",
    "/etc/resolv.conf": "file",
    "/etc/hosts":       "file",
    "/etc/hostname":    "file",
    "/dev/console":     "file",
    "/etc/mtab":        "/proc/mounts",

code link

Answer 2

<container>-init layer exists to create certain must exist files/directories(usually as mountpoints), so that docker can bind mount to these mountpoints without worrying they don't exist.

NOTE: init layer is RO(READONLY), can not by modified.

These mountpoints are usually empty, and the following explains their purpose:

• /proc: in-memory data about processes and system
• sys: in-memory system file system
• /etc/hostname: container hostname file, each container will have their own hostname, and bind mount to this file
• /etc/hosts: hosts file, ip and name mapping
• /etc/resolv.conf: DNS-related resolve conf file
• ......