Why docker debian image show Ubuntu as OS?

I did pull from dockerhub:

docker pull mysql

In its Dockerfile, I see:

FROM debian:stretch-slim

But when I run the container:

sudo docker run -it mysql /bin/bash

And in the container I write the command:

uname -a

and the output is:

Linux 050e1a4b22ed 4.18.0-18-generic #19-Ubuntu SMP Tue Apr 2 18:13:16 UTC 2019 x86_64 GNU/Linux

Why the OS is Ubuntu if the base image in the dockerfile is debian:stretch-slim?

Answer

Docker containers use the kernel of their host, and your host is running an Ubuntu kernel.

Containers are isolated instances created by OS-level virtualization. On Linux, containers run on the kernel of the host, so your container's uname kernel release and version output is going to be same as the host's.

---

The host is running Ubuntu 18.04 with kernel release 4.18.0-18-generic, and the OS-level virtualization implementation is LXC (similar to Docker).

deltik@host [~]$ lxc launch -s local images:debian/10 demo

Creating demo
Starting demo
deltik@host [~]$ uname -a
Linux host 4.18.0-18-generic #19~18.04.1-Ubuntu SMP Fri Apr 5 10:22:13 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Debian 10 Container

deltik@host [~]$ lxc exec demo -- bash

root@demo:~# cat /etc/*-release
PRETTY_NAME="Debian GNU/Linux buster/sid"
NAME="Debian GNU/Linux"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
root@demo:~# uname -a
Linux demo 4.18.0-18-generic #19~18.04.1-Ubuntu SMP Fri Apr 5 10:22:13 UTC 2019 x86_64 GNU/Linux

CentOS 7 Container

deltik@host [~]$ lxc launch -s local images:centos/7 demo
Creating demo
Starting demo
deltik@host [~]$ lxc exec demo -- bash
[root@demo ~]# cat /etc/*-release
CentOS Linux release 7.6.1810 (Core) 

NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"


CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

CentOS Linux release 7.6.1810 (Core) 
CentOS Linux release 7.6.1810 (Core) 
[root@demo ~]# uname -a
Linux demo 4.18.0-18-generic #19~18.04.1-Ubuntu SMP Fri Apr 5 10:22:13 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Gentoo Container

deltik@host [~]$ lxc launch -s local images:gentoo demo
Creating demo
Starting demo
deltik@host [~]$ lxc exec demo -- bash
demo ~ # cat /etc/*-release

Gentoo Base System release 2.6
NAME=Gentoo
ID=gentoo
PRETTY_NAME="Gentoo/Linux"
ANSI_COLOR="1;32"
HOME_URL="https://www.gentoo.org/"
SUPPORT_URL="https://www.gentoo.org/support/"
BUG_REPORT_URL="https://bugs.gentoo.org/"
demo ~ # uname -a
Linux demo 4.18.0-18-generic #19~18.04.1-Ubuntu SMP Fri Apr 5 10:22:13 UTC 2019 x86_64 Intel(R) Xeon(R) E-2136 CPU @ 3.30GHz GenuineIntel GNU/Linux