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Build your own image

The whalesay image could be improved. It would be nice if you didn’t have to think of something to say. And you type a lot to get whalesay to talk.

docker run docker/whalesay cowsay boo-boo

In this next section, you will improve the whalesay image by building a new version that “talks on its own” and requires fewer words to run.

Step 1: Write a Dockerfile

In this step, you use your favorite text editor to write a short Dockerfile. A Dockerfile describes the software that is “baked” into an image. It isn’t just ingredients tho, it can tell the software what environment to use or what commands to run. Your recipe is going to be very short.

  1. Go back to your command terminal window.

  2. Make a new directory by typing mkdir mydockerbuild and pressing RETURN.

    $ mkdir mydockerbuild

    This directory serves as the “context” for your build. The context just means it contains all the things you need to build your image.

  3. Change to your new directory.

    $ cd mydockerbuild

    Right now the directory is empty.

  4. Create a Dockerfile in the directory by typing touch Dockerfile and pressing RETURN.

    $ touch Dockerfile

    The command appears to do nothing but it actually creates the Dockerfile in the current directory. Just type ls Dockerfile to see it.

    $ ls Dockerfile
  5. Open the Dockerfile in a visual text editor like Atom or Sublime, or a text based editor like vi, or nano (

  6. Add a line to the file like this:

    FROM docker/whalesay:latest

    The FROM keyword tells Docker which image your image is based on. Whalesay is cute and has the cowsay program already, so we’ll start there.

  7. Now, add the fortunes program to the image.

    RUN apt-get -y update && apt-get install -y fortunes

    The fortunes program has a command that prints out wise sayings for our whale to say. So, the first step is to install it. This line installs the software into the image.

  8. Once the image has the software it needs, you instruct the software to run when the image is loaded.

      CMD /usr/games/fortune -a | cowsay

    This line tells the fortune program to pass a nifty quote to the cowsay program.

  9. Check your work, your file should look like this:

    FROM docker/whalesay:latest
    RUN apt-get -y update && apt-get install -y fortunes
    CMD /usr/games/fortune -a | cowsay
  10. Save and close your Dockerfile.

    At this point, you have all your software ingredients and behaviors described in a Dockerfile. You are ready to build a new image.

Step 2: Build an image from your Dockerfile

  1. At the command line, make sure the Dockerfile is in the current directory by typing cat Dockerfile

    $ cat Dockerfile
    FROM docker/whalesay:latest
    RUN apt-get -y update && apt-get install -y fortunes
    CMD /usr/games/fortune -a | cowsay
  2. Now, build your new image by typing the docker build -t docker-whale . command in your terminal (don’t forget the . period).

    $ docker build -t docker-whale .
    Sending build context to Docker daemon 158.8 MB
    Removing intermediate container a8e6faa88df3
    Successfully built 7d9495d03763

    The command takes several seconds to run and reports its outcome. Before you do anything with the new image, take a minute to learn about the Dockerfile build process.

Step 3: Learn about the build process

The docker build -t docker-whale . command takes the Dockerfile in the current directory, and builds an image called docker-whale on your local machine. The command takes about a minute and its output looks really long and complex. In this section, you learn what each message means.

First Docker checks to make sure it has everything it needs to build.

Sending build context to Docker daemon 158.8 MB

Then, Docker loads with the whalesay image. It already has this image locally as you might recall from the last page. So, Docker doesn’t need to download it.

Step 0 : FROM docker/whalesay:latest
 ---> fb434121fc77

Docker moves onto the next step which is to update the apt-get package manager. This takes a lot of lines, no need to list them all again here.

Step 1 : RUN apt-get -y update && apt-get install -y fortunes
 ---> Running in 27d224dfa5b2
Ign trusty InRelease
Ign trusty-updates InRelease
Ign trusty-security InRelease
Hit trusty Release.gpg
Get:15 trusty-security/restricted amd64 Packages [14.8 kB]
Get:16 trusty-security/universe amd64 Packages [134 kB]
Reading package lists...
---> eb06e47a01d2

Then, Docker installs the new fortunes software.

Removing intermediate container e2a84b5f390f
Step 2 : RUN apt-get install -y fortunes
 ---> Running in 23aa52c1897c
Reading package lists...
Building dependency tree...
Reading state information...
The following extra packages will be installed:
  fortune-mod fortunes-min librecode0
Suggested packages:
  x11-utils bsdmainutils
The following NEW packages will be installed:
  fortune-mod fortunes fortunes-min librecode0
0 upgraded, 4 newly installed, 0 to remove and 3 not upgraded.
Need to get 1961 kB of archives.
After this operation, 4817 kB of additional disk space will be used.
Get:1 trusty/main librecode0 amd64 3.6-21 [771 kB]
Setting up fortunes (1:1.99.1-7) ...
Processing triggers for libc-bin (2.19-0ubuntu6.6) ...
 ---> c81071adeeb5
Removing intermediate container 23aa52c1897c

Finally, Docker finishes the build and reports its outcome.

Step 3 : CMD /usr/games/fortune -a | cowsay
 ---> Running in a8e6faa88df3
 ---> 7d9495d03763
Removing intermediate container a8e6faa88df3
Successfully built 7d9495d03763

Step 4: Run your new docker-whale

In this step, you verify the new images is on your computer and then you run your new image.

  1. Open a command line terminal.

  2. Type docker images and press RETURN.

    This command, you might remember, lists the images you have locally.

    $ docker images
    REPOSITORY           TAG          IMAGE ID          CREATED             VIRTUAL SIZE
    docker-whale         latest       7d9495d03763      4 minutes ago       273.7 MB
    docker/whalesay      latest       fb434121fc77      4 hours ago         247 MB
    hello-world          latest       91c95931e552      5 weeks ago         910 B
  3. Run your new image by typing docker run docker-whale and pressing RETURN.

    $ docker run docker-whale
    / "He was a modest, good-humored boy. It  \
    \ was Oxford that made him insufferable." /
                              ##        .            
                        ## ## ##       ==            
                     ## ## ## ##      ===            
                 /""""""""""""""""___/ ===        
            ~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ /  ===- ~~~   
                 \______ o          __/            
                  \    \        __/             

As you can see, you’ve made the whale a lot smarter. It finds its own things to say and the command line is a lot shorter! You may also notice that Docker didn’t have to download anything. That is because the image was built locally and is already available.

Where to go next

On this page, you learned to build an image by writing your own Dockerfile. You ran your image in a container. You also just used Linux from your Mac yet again. In the next section, you take the first step in sharing your image by creating a Docker Hub account.