Docker Quick Start
As explained in another post, Docker is software designed to ‘containerize’ applications. While it can be complex to implement on a larger scale, it is quite easy to quickly launch and test on a single machine.
Environment
Containers are platform-independent - a major boost to portability in containerized applications.
However, to exactly replicate these steps, here are the specifications:
- CentOS 8
docker-ce-3:19.03.5-3.el7.x86_64
These steps will largely be adapted from the official CentOS/Docker documentation.
Prerequisites
Begin by installing some utilities needed to setup our docker yum repository, and
some drivers needed by the docker engine.
[mehlj@docker ~]$ sudo yum install -y yum-utils device-mapper-persistent-data lvm2
Last metadata expiration check: 0:11:58 ago on Tue 21 Jan 2020 07:45:35 PM EST.
Package device-mapper-persistent-data-0.8.5-2.el8.x86_64 is already installed.
Package lvm2-8:2.03.05-5.el8.0.1.x86_64 is already installed.
Dependencies resolved.
==========================================================================================================================================================================================================================================================================================
Package Architecture Version Repository Size
==========================================================================================================================================================================================================================================================================================
Installing:
yum-utils noarch 4.0.8-3.el8 BaseOS 64 k
Transaction Summary
==========================================================================================================================================================================================================================================================================================
Install 1 Package
Total download size: 64 k
Installed size: 19 k
Downloading Packages:
yum-utils-4.0.8-3.el8.noarch.rpm 399 kB/s | 64 kB 00:00
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total 154 kB/s | 64 kB 00:00
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
Preparing : 1/1
Installing : yum-utils-4.0.8-3.el8.noarch 1/1
Running scriptlet: yum-utils-4.0.8-3.el8.noarch 1/1
Verifying : yum-utils-4.0.8-3.el8.noarch 1/1
Installed:
yum-utils-4.0.8-3.el8.noarch
Complete!
[mehlj@docker ~]$
Add the stable repository for docker to our machine:
[mehlj@docker ~]$ sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
Adding repo from: https://download.docker.com/linux/centos/docker-ce.repo
[mehlj@docker ~]$
Install the docker engine and containerd. Note that the --nobest flag must be passed, as there was, at the time
of writing this, a dependency issue with CentOS 8 and containerd.
[mehlj@docker ~]$ sudo yum install docker-ce docker-ce-cli containerd.io --nobest -y
Last metadata expiration check: 0:00:27 ago on Tue 21 Jan 2020 08:14:11 PM EST.
Dependencies resolved.
Problem: package docker-ce-3:19.03.5-3.el7.x86_64 requires containerd.io >= 1.2.2-3, but none of the providers can be installed
- cannot install the best candidate for the job
- package containerd.io-1.2.10-3.2.el7.x86_64 is excluded
- package containerd.io-1.2.2-3.3.el7.x86_64 is excluded
- package containerd.io-1.2.2-3.el7.x86_64 is excluded
- package containerd.io-1.2.4-3.1.el7.x86_64 is excluded
- package containerd.io-1.2.5-3.1.el7.x86_64 is excluded
- package containerd.io-1.2.6-3.3.el7.x86_64 is excluded
==========================================================================================================================================================================================================================================================================================
Package Architecture Version Repository Size
==========================================================================================================================================================================================================================================================================================
Installing:
containerd.io x86_64 1.2.0-3.el7 docker-ce-stable 22 M
docker-ce x86_64 3:18.09.1-3.el7 docker-ce-stable 19 M
docker-ce-cli x86_64 1:19.03.5-3.el7 docker-ce-stable 39 M
Installing dependencies:
libcgroup x86_64 0.41-19.el8 BaseOS 70 k
Skipping packages with broken dependencies:
docker-ce x86_64 3:19.03.5-3.el7 docker-ce-stable 24 M
Transaction Summary
==========================================================================================================================================================================================================================================================================================
Install 4 Packages
Skip 1 Package
Total size: 80 M
Installed size: 338 M
Downloading Packages:
[SKIPPED] libcgroup-0.41-19.el8.x86_64.rpm: Already downloaded
[SKIPPED] containerd.io-1.2.0-3.el7.x86_64.rpm: Already downloaded
[SKIPPED] docker-ce-18.09.1-3.el7.x86_64.rpm: Already downloaded
[SKIPPED] docker-ce-cli-19.03.5-3.el7.x86_64.rpm: Already downloaded
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
Preparing : 1/1
Installing : docker-ce-cli-1:19.03.5-3.el7.x86_64 1/4
Running scriptlet: docker-ce-cli-1:19.03.5-3.el7.x86_64 1/4
Installing : containerd.io-1.2.0-3.el7.x86_64 2/4
Running scriptlet: containerd.io-1.2.0-3.el7.x86_64 2/4
Running scriptlet: libcgroup-0.41-19.el8.x86_64 3/4
Installing : libcgroup-0.41-19.el8.x86_64 3/4
Running scriptlet: libcgroup-0.41-19.el8.x86_64 3/4
Running scriptlet: docker-ce-3:18.09.1-3.el7.x86_64 4/4
Installing : docker-ce-3:18.09.1-3.el7.x86_64 4/4
Running scriptlet: docker-ce-3:18.09.1-3.el7.x86_64 4/4
Verifying : libcgroup-0.41-19.el8.x86_64 1/4
Verifying : containerd.io-1.2.0-3.el7.x86_64 2/4
Verifying : docker-ce-3:18.09.1-3.el7.x86_64 3/4
Verifying : docker-ce-cli-1:19.03.5-3.el7.x86_64 4/4
Installed:
containerd.io-1.2.0-3.el7.x86_64 docker-ce-3:18.09.1-3.el7.x86_64 docker-ce-cli-1:19.03.5-3.el7.x86_64 libcgroup-0.41-19.el8.x86_64
Skipped:
docker-ce-3:19.03.5-3.el7.x86_64
Complete!
[mehlj@docker ~]$
Start the docker service:
[mehlj@docker ~]$ sudo service docker start
Redirecting to /bin/systemctl start docker.service
Testing
Now that docker is installed, we can use a proof-of-concept image from Docker Hub known as hello-world to verify
our installation. Docker Hub is a public collection of pre-made image shared by the community. This hello-world
container is provided by the company Docker itself.
To run this container, we simple need to use docker run:
[mehlj@docker ~]$ sudo docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
1b930d010525: Pull complete
Digest: sha256:9572f7cdcee8591948c2963463447a53466950b3fc15a247fcad1917ca215a2f
Status: Downloaded newer image for hello-world:latest
Hello from Docker!
This message shows that your installation appears to be working correctly.
To generate this message, Docker took the following steps:
1. The Docker client contacted the Docker daemon.
2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
(amd64)
3. The Docker daemon created a new container from that image which runs the
executable that produces the output you are currently reading.
4. The Docker daemon streamed that output to the Docker client, which sent it
to your terminal.
To try something more ambitious, you can run an Ubuntu container with:
$ docker run -it ubuntu bash
Share images, automate workflows, and more with a free Docker ID:
https://hub.docker.com/
For more examples and ideas, visit:
https://docs.docker.com/get-started/
[mehlj@docker ~]$
As we notice from the output, docker first checks if the image was present on the local machine:
Unable to find image 'hello-world:latest' locally
After it determines that the image is not present locally, it will automatically download the image from Docker Hub.
Once it downloads, it runs in the docker engine.
As the output states, we can try something more ambitious by running a full Ubuntu container, instead of the simple
hello-world container:
[mehlj@docker ~]$ sudo docker run -it ubuntu bash
Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu
5c939e3a4d10: Pull complete
c63719cdbe7a: Pull complete
19a861ea6baf: Pull complete
651c9d2d6c4f: Pull complete
Digest: sha256:8d31dad0c58f552e890d68bbfb735588b6b820a46e459672d96e585871acc110
Status: Downloaded newer image for ubuntu:latest
root@8efc1921398e:/# uname -a
Linux 8efc1921398e 4.18.0-147.3.1.el8_1.x86_64 #1 SMP Fri Jan 3 23:55:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
root@8efc1921398e:/#
The container was created from the image simply known as ubuntu, and our flags -it and our call to bash allowed
us to enter an interactive terminal session with it.
From this, we can determine that our Docker installation is working as intended, and this system is ready for more practical usage of Docker.