In this section, you will create your first container image. Please start by creating an empty directory and change into this directory
mkdir first_image
cd first_image
In this directory you will create the Dockerfile for your first image. Copy and paste the following content:
FROM ubuntu:18.04
RUN apt-get update && apt-get install -y cowsay
CMD /usr/games/cowsay "Containers are awesome!"Now, from within directory containing the Dockerfile, you can build the container image:
docker image build -t your_docker_id/cowsay:v1 .
The output of the command will look similar to this (note that you might not see the "Pulling from …" part if you followed the getting started part).:
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM ubuntu:18.04
18.04: Pulling from library/ubuntu
6cf436f81810: Pull complete
987088a85b96: Pull complete
b4624b3efe06: Pull complete
d42beb8ded59: Pull complete
Digest: sha256:7a47ccc3bbe8a451b500d2b53104868b46d60ee8f5b35a24b41a86077c650210
Status: Downloaded newer image for ubuntu:18.04
---> 47b19964fb50
Step 2/3 : RUN apt-get update && apt-get install -y cowsay
---> Running in 28d176829ae1
Get:1 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:2 http://archive.ubuntu.com/ubuntu bionic InRelease [242 kB]
Get:3 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages [339 kB]
Get:4 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:5 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Get:6 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages [11.3 MB]
Get:7 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 Packages [3451 B]
Get:8 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [147 kB]
Get:9 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages [1344 kB]
Get:10 http://archive.ubuntu.com/ubuntu bionic/restricted amd64 Packages [13.5 kB]
Get:11 http://archive.ubuntu.com/ubuntu bionic/multiverse amd64 Packages [186 kB]
Get:12 http://archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 Packages [6955 B]
Get:13 http://archive.ubuntu.com/ubuntu bionic-updates/restricted amd64 Packages [10.7 kB]
Get:14 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages [929 kB]
Get:15 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages [679 kB]
Get:16 http://archive.ubuntu.com/ubuntu bionic-backports/universe amd64 Packages [4690 B]
Fetched 15.5 MB in 6s (2466 kB/s)
Reading package lists...
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
libgdbm-compat4 libgdbm5 libperl5.26 libtext-charwidth-perl netbase perl
perl-modules-5.26
Suggested packages:
filters cowsay-off gdbm-l10n perl-doc libterm-readline-gnu-perl
| libterm-readline-perl-perl make
The following NEW packages will be installed:
cowsay libgdbm-compat4 libgdbm5 libperl5.26 libtext-charwidth-perl netbase
perl perl-modules-5.26
0 upgraded, 8 newly installed, 0 to remove and 2 not upgraded.
Need to get 6563 kB of archives.
After this operation, 41.7 MB of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 perl-modules-5.26 all 5.26.1-6ubuntu0.3 [2763 kB]
Get:2 http://archive.ubuntu.com/ubuntu bionic/main amd64 libgdbm5 amd64 1.14.1-6 [26.0 kB]
Get:3 http://archive.ubuntu.com/ubuntu bionic/main amd64 libgdbm-compat4 amd64 1.14.1-6 [6084 B]
Get:4 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 libperl5.26 amd64 5.26.1-6ubuntu0.3 [3527 kB]
[... SNIP ...]
Unpacking cowsay (3.03+dfsg2-4) ...
Setting up perl-modules-5.26 (5.26.1-6ubuntu0.3) ...
Setting up libgdbm5:amd64 (1.14.1-6) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Setting up libtext-charwidth-perl (0.04-7.1) ...
Setting up libgdbm-compat4:amd64 (1.14.1-6) ...
Setting up netbase (5.4) ...
Setting up libperl5.26:amd64 (5.26.1-6ubuntu0.3) ...
Setting up perl (5.26.1-6ubuntu0.3) ...
Setting up cowsay (3.03+dfsg2-4) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
Removing intermediate container 28d176829ae1
---> c14904ec9828
Step 3/3 : CMD /usr/games/cowsay "Containers are awesome!"
---> Running in 487bea9e4c74
Removing intermediate container 487bea9e4c74
---> 760f3018b10b
Successfully built 760f3018b10b
Successfully tagged your_docker_id/cowsay:v1
After the build process finishes, you can verify that the build was successful and that image exists on your system:
docker image ls # to list all images
docker image ls your_docker_id/cowsay # to list images in the `your_docker_id/cowsay` repository
We can now run a container based on this image:
docker container run your_docker_id/cowsay:v1
You should see they cow saying Containers are awesome!:
_________________________
< Containers are awesome! >
-------------------------
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||
Time to play around for a bit! We can execute arbitrary commands in a container - all we have to do is to append the command after the image name. For example:
docker container run your_docker_id/cowsay:v1 echo "Running the echo command in the cowsay image"
This will print Running the echo command in the cowsay image. The output was produced by the echo executable that is part of the your_docker_id/cowsay:v1 image. In fact, we "inherited" this executable from the ubuntu:18.04 image.
The executable for whatever command we run in the container must be present in the container image. This means even with Ruby installed on our local system, the following command will fail:
docker container run your_docker_id/cowsay:v1 ruby -v
Let's try something else and start an interactive program like a shell in a container. We already know that we can specify which command we want to run in the container. However, for interactive applications like a shell, we have to specify the -it flags before the image name:
docker container run -it your_docker_id/cowsay:v1 bash
This command will open a shell prompt in a new container. Try it out - you will be able to navigate the containers file system and execute arbitrary commands as well. For example:
uname -a
ls -l /usr/bin
whoami
Once you are done, you can exit the shell and quit the container by pressing Ctrl-D
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