tp-docker-6.md

Creating Images with Dockefiles

Writing and Building a Dockerfile

1. Create a folder called myimage, and a text file called Dockerfile within that folder. In Dockerfile, include the following instructions:

FROM centos:7
RUN yum update -y
RUN yum install -y wget

This serves as a recipe for an image based on centos:7, that has all its default packages updated and wget installed on top.

2. Build your image with the build command:

$ docker image build -t myimage .

You’ll see a long build output - we’ll go through the meaning of this output in a demo later. For now, everything is good if it ends with Successfully tagged myimage:latest.

3. Verify that your new image exists with docker image ls, then use it to run a container and wget something from within that container.

4. It’s also possible to pipe a Docker le in from STDIN; try rebuilding your image with the following:

$ cat Dockerfile | docker image build -t myimage -f - .

(This is useful when reading a Docker le from a remote location with curl, for example).

Using the Build Cache

In the previous step, the second time you built your image should have completed immediately, with each step save the first reporting using cache. Cached build steps will be used until a change in the Dockerfile is found by the builder.

• Open your Docker le and add another RUN step at the end to install vim
• Build the image again as above; which steps is the cache used for?
• Build the image again; which steps use the cache this time?
• Swap the order of the two RUN commands for installing wget and vim in the Dockerfile, and build one last time. Which steps are cached this time?

Using the history Command

1. The docker image history command allows us to inspect the build cache history of an image. Try it with your new image:

$ docker image history myimage:latest

Note the image id of the layer built for the yum update command.

2. Replace the two RUN commands that installed wget and vim with a single command:

...
RUN yum install -y wget vim

3. Build the image again, and run docker image history on this new image. How has the history changed?

In this exercise, we’ve seen how to write a basic Docker le using FROM and RUN commands, some basics of how image caching works, and seen the docker image history command. Using the build cache e ectively is crucial for images that involve lengthy compile or download steps; in general, moving commands that change frequently as late as possible in the Docker le will minimize build times. We’ll see some more speci c advice on this later in this lesson.