Obtain a free account at your favorite hosted Container Registry, Docker Hub, Quay or others. In the following examples, we will be using the most popular registry Docker Hub, which is the default with the docker cli & runtime. For an overview and comparison of public/hosted/private/on-premise registry options check link 1 and link 2
We will upload/push our container image into a public repository. First, we login to the registry; this is only required once, the docker cli saves your authorization credentials under ~/.docker/config.json. Next, we tag the image with a url tag docker.io/vasu1124/container:1.0 or for Docker Hub in short vasu1124/container:1.0; for other registries you have to use an absolute url. Lastly, if everything works, the image is pushed.
# docker tag container:1.0 vasu1124/container:1.0
# docker login
Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com to create one.
Username: vasu1124
Password: <password>
Login Succeeded
# docker push vasu1124/container:1.0
The push refers to a repository [docker.io/vasu1124/container]
5b73e1d6f17e: Pushed
1.0: digest: sha256:c668d2784a267f29ee6c397537c48454c3dba829da2f513b37a5fafd16975fac size: 527
If you now open the Browser to https://hub.docker.com/r/vasu1124/container/tags/, you will find our container image publicly accessible.
How about logging into a fresh system with docker installed and running docker run -it vasu1124/container:1.0 /bin/bash? If you do, you have just experienced the container benefits of standardized packaging & distribution. No need for complex installations & scripting.
The container image can now be used and/or extended by a community of users for which this image might be of interest. Beware that you are responsible for ensuring copyrights, licenses and export controls are observed. All registry services offer multiple advanced, mostly paid features, ranging from integrating source to image CI/CD build pipelines to security & vulnerability scanners (see the Ubuntu example).
It is now time to tap into the collective contributions of a decentralized, self-organized community and search for an image you can directly use (instead of building it from scratch):
# docker search centos
NAME DESCRIPTION STARS OFFICIAL AUTOMATED
centos The official build of CentOS. 4101 [OK]
ansible/centos7-ansible Ansible on Centos7 105 [OK]
jdeathe/centos-ssh CentOS-6 6.9 x86_64 / CentOS-7 7.4.1708 x8... 93 [OK]
consol/centos-xfce-vnc Centos container with "headless" VNC sessi... 48 [OK]
imagine10255/centos6-lnmp-php56 centos6-lnmp-php56 40 [OK]
tutum/centos Simple CentOS docker image with SSH access 36
gluster/gluster-centos Official GlusterFS Image [ CentOS-7 + Glu... 25 [OK]
centos/python-35-centos7 Platform for building and running Python 3... 19
...
smartentry/centos centos with smartentry 0 [OK]
openshift/wildfly-81-centos7 A Centos7 based WildFly v8.1 image for use... 0
# docker search gentoo
NAME DESCRIPTION STARS OFFICIAL AUTOMATED
gentoo/stage3-amd64 65 [OK]
gentoo/portage 30 [OK]
gentoo/stage3-amd64-hardened Official Gentoo stage3-amd64-hardened Image 17 [OK]
gentoo/stage3-amd64-nomultilib Official Gentoo stage3-amd64-nomultilib Image 14 [OK]
plabedan/gentoo-minimal Gentoo minimal image (stage3 + portage) 7
You can now choose from various options. Docker Hub offers some KPIs, such as how many "Stars" has the image obtained from the users, is it an "official" build from docker, or is an "automated" build process attached.
First, let us inspect the host system:
# cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.4 LTS (Xenial Xerus)"
ID=ubuntu
...
# uname -a
Linux ubuntu-xenial 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
So this is an Ubuntu with an Ubuntu kernel.
Now, let us download, run and inspect the official centos:7 image:
# docker run -it centos:7 /bin/bash
Unable to find image 'centos:7' locally
7: Pulling from library/centos
5e35d10a3eba: Pull complete
Digest: sha256:dcbc4e5e7052ea2306eed59563da1fec09196f2ecacbe042acbdcd2b44b05270
Status: Downloaded newer image for centos:7
[root@f6ebd1b19515 /]# cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
...
[root@f6ebd1b19515 /]# ls -la
[snip ... looks like a full operating system!]
[root@f6ebd1b19515 /]# uname -a
Linux f6ebd1b19515 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
[root@f6ebd1b19515 /]# exit
Woa. Inside the container, it feels like a wholehearted CentOS. But the kernel is an Ubuntu? ... Get used to it. This is the new world of containers, your RHEL based image might run on a SLES kernel and vice versa, your support matrix just got hyperboled :-)
Where you can import, you can also export (need to keep the trade balance in check):
# mkdir centos
# cd centos; docker export f6ebd1b19515 | tar xf -
Your local folder ~/centos now includes a complete CentOS v7 root filesystem. If you figure out where to get kernel & initrd from and how to appropriatly boot into that system, you have an installer for an Operating System.
Checkout https://hub.docker.com/explore/ and more recently https://store.docker.com/ for curated and well maintained Docker images with many open source and commercial software products.
Lastly, there are two CNCF incubating projects dealing with managing cryptographic based trust and integrity of image & contents: The Update Framework (TUF) and Notary.