What's the container of a base docker image?

Question

I am learning to use docker. I know that each docker image is build on a base image which doesn't have a parent.

Then from a base image, we can customize some stuff via containers (maybe a very short life container) to commit a new image.

So I understand that the process is like this: base image -> container -> new image 1 -> container -> new image 2

However, I inspect the json data of the base image. I still can see that it has a container's information:

[{
"Architecture": "amd64",
"Author": "",
"Comment": "",
"Config": {
    "AttachStderr": false,
    "AttachStdin": false,
    "AttachStdout": false,
    "Cmd": null,
    "CpuShares": 0,
    "Cpuset": "",
    "Domainname": "",
    "Entrypoint": null,
    "Env": null,
    "ExposedPorts": null,
    "Hostname": "3f37dbc61890",
    "Image": "",
    "Labels": null,
    "MacAddress": "",
    "Memory": 0,
    "MemorySwap": 0,
    "NetworkDisabled": false,
    "OnBuild": null,
    "OpenStdin": false,
    "PortSpecs": null,
    "StdinOnce": false,
    "Tty": false,
    "User": "",
    "Volumes": null,
    "WorkingDir": ""
},
"Container": "3f37dbc61890b0bb37cc8479db94602bcc2d6e177d76c0f3d7d53346c0dc580c",
"ContainerConfig": {
    "AttachStderr": false,
    "AttachStdin": false,
    "AttachStdout": false,
    "Cmd": [
        "/bin/sh",
        "-c",
        "#(nop) ADD file:777fad733fc954c0c161670c48c10ea1787a6e5d544daa20e55d593279df3fa3 in /"
    ],
    "CpuShares": 0,
    "Cpuset": "",
    "Domainname": "",
    "Entrypoint": null,
    "Env": null,
    "ExposedPorts": null,
    "Hostname": "3f37dbc61890",
    "Image": "",
    "Labels": null,
    "MacAddress": "",
    "Memory": 0,
    "MemorySwap": 0,
    "NetworkDisabled": false,
    "OnBuild": null,
    "OpenStdin": false,
    "PortSpecs": null,
    "StdinOnce": false,
    "Tty": false,
    "User": "",
    "Volumes": null,
    "WorkingDir": ""
},
"Created": "2015-04-21T22:18:45.67739694Z",
"DockerVersion": "1.6.0",
"Id": "706766fe101906a1a6628173c2677173a5f8c6c469075083f3cf3a8f5e5eb367",
"Os": "linux",
"Parent": "",
"Size": 188104128,
"VirtualSize": 188104128
}]

3f37dbc61890b0bb37cc8479db94602bcc2d6e177d76c0f3d7d53346c0dc580c is the container id of the base image.

what is a container of a base image? I feel like it becomes the hostname. What is the hostname actually?

Answer 1

I'm not sure of your terminology. If a base image is by definition an image that has no parent, then the image in your example is not a base image.

But an image may have no parent. scratch , for example, has no parent:

$ docker inspect -f '{{.Container}}' ubuntu
a4c15f8c80978475a53f96721f935de5823bc8c29aff14eb00a15f9b9d96cddd
$ docker inspect -f '{{.Container}}' scratch

$

You can also create an image that has no parent using import :

$ echo hello world > foo && tar -cf- foo | docker import -
3e8fc0cb69fae0bd3f9711031df6d3b7bf6a7e8c9745657d9261e7b803718c67
$ docker inspect -f '{{.Container}}' 3e8fc0c

$

Unlike scratch , this image may have files in it. In fact, you can flatten a complex image using this technique.

$ docker create ubuntu # create container from image
6b90bf145c193ef8e4ecb789372d2fd619769a20d96c8f3f586dcfbc501b0611
$ docker export 6b90bf1 > ubuntu.tar # export container fs to tarball
$ docker import - flat_ubuntu < ubuntu.tar
4ef4ffb9514212acf6a19b2eeda8855b8c0445924311043ba5cba6574d40d772
$ docker inspect -f '{{.Container}}' 4ef4ffb

$

It's important to note that while this new image has the exact same files as the original image, it doesn't have the other docker features like environment, volume, entrypoint, etc.

I would not necessarily call this a "base" image. I would call it a "flat" image. I would say a base image is the image you indicate in a Dockerfile FROM directive. In my terminology a base image need not be flat.