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168 lines
3.2 KiB
Markdown
168 lines
3.2 KiB
Markdown
# Building Images Interactively
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In this section, we will create our first container image.
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It will be a basic distribution image, but we will pre-install
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the package `figlet`.
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We will:
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* Create a container from a base image.
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* Install software manually in the container, and turn it
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into a new image.
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* Learn about new commands: `docker commit`, `docker tag`, and `docker diff`.
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---
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## Building Images Interactively
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As we have seen, the images on the Docker Hub are sometimes very basic.
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How do we want to construct our own images?
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As an example, we will build an image that has `figlet`.
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First, we will do it manually with `docker commit`.
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Then, in an upcoming chapter, we will use a `Dockerfile` and `docker build`.
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---
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## Building from a base
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Our base will be the `ubuntu` image.
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---
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## Create a new container and make some changes
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Start an Ubuntu container:
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```bash
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$ docker run -it ubuntu
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root@<yourContainerId>:#/
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```
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Run the command `apt-get update` to refresh the list of packages available to install.
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Then run the command `apt-get install figlet` to install the program we are interested in.
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```bash
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root@<yourContainerId>:#/ apt-get update && apt-get install figlet
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.... OUTPUT OF APT-GET COMMANDS ....
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```
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---
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## Inspect the changes
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Type `exit` at the container prompt to leave the interactive session.
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Now let's run `docker diff` to see the difference between the base image
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and our container.
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```bash
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$ docker diff <yourContainerId>
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C /root
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A /root/.bash_history
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C /tmp
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C /usr
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C /usr/bin
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A /usr/bin/figlet
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...
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```
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---
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class: x-extra-details
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## Docker tracks filesystem changes
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As explained before:
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* An image is read-only.
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* When we make changes, they happen in a copy of the image.
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* Docker can show the difference between the image, and its copy.
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* For performance, Docker uses copy-on-write systems.
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<br/>(i.e. starting a container based on a big image
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doesn't incur a huge copy.)
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---
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## Copy-on-write security benefits
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* `docker diff` gives us an easy way to audit changes
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(à la Tripwire)
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* Containers can also be started in read-only mode
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(their root filesystem will be read-only, but they can still have read-write data volumes)
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---
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## Commit and run your image
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The `docker commit` command will create a new layer with those changes,
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and a new image using this new layer.
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```bash
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$ docker commit <yourContainerId>
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<newImageId>
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```
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The output of the `docker commit` command will be the ID for your newly created image.
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We can run this image:
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```bash
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$ docker run -it <newImageId>
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root@fcfb62f0bfde:/# figlet hello
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_ _ _
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| |__ ___| | | ___
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| '_ \ / _ \ | |/ _ \
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| | | | __/ | | (_) |
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|_| |_|\___|_|_|\___/
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```
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---
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## Tagging images
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Referring to an image by its ID is not convenient. Let's tag it instead.
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We can use the `tag` command:
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```bash
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$ docker tag <newImageId> figlet
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```
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But we can also specify the tag as an extra argument to `commit`:
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```bash
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$ docker commit <containerId> figlet
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```
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And then run it using its tag:
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```bash
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$ docker run -it figlet
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```
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---
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## What's next?
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Manual process = bad.
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Automated process = good.
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In the next chapter, we will learn how to automate the build
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process by writing a `Dockerfile`.
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