class: title
# Building Docker images with a Dockerfile
---
## Objectives
We will build a container image automatically, with a `Dockerfile`.
At the end of this lesson, you will be able to:
* Write a `Dockerfile`.
* Build an image from a `Dockerfile`.
---
## `Dockerfile` overview
* A `Dockerfile` is a build recipe for a Docker image.
* It contains a series of instructions telling Docker how an image is constructed.
* The `docker build` command builds an image from a `Dockerfile`.
---
## Writing our first `Dockerfile`
Our Dockerfile must be in a **new, empty directory**.
1. Create a directory to hold our `Dockerfile`.
```bash
$ mkdir myimage
```
2. Create a `Dockerfile` inside this directory.
```bash
$ cd myimage
$ vim Dockerfile
```
Of course, you can use any other editor of your choice.
---
## Type this into our Dockerfile...
```dockerfile
FROM ubuntu
RUN apt-get update
RUN apt-get install figlet
```
* `FROM` indicates the base image for our build.
* Each `RUN` line will be executed by Docker during the build.
* Our `RUN` commands **must be non-interactive.**
(No input can be provided to Docker during the build.)
* In many cases, we will add the `-y` flag to `apt-get`.
---
## Build it!
Save our file, then execute:
```bash
$ docker build -t figlet .
```
* `-t` indicates the tag to apply to the image.
* `.` indicates the location of the *build context*.
We will talk more about the build context later.
To keep things simple for now: this is the directory where our Dockerfile is located.
---
## What happens when we build the image?
The output of `docker build` looks like this:
.small[
```bash
docker build -t figlet .
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM ubuntu
---> f975c5035748
Step 2/3 : RUN apt-get update
---> Running in e01b294dbffd
(...output of the RUN command...)
Removing intermediate container e01b294dbffd
---> eb8d9b561b37
Step 3/3 : RUN apt-get install figlet
---> Running in c29230d70f9b
(...output of the RUN command...)
Removing intermediate container c29230d70f9b
---> 0dfd7a253f21
Successfully built 0dfd7a253f21
Successfully tagged figlet:latest
```
]
* The output of the `RUN` commands has been omitted.
* Let's explain what this output means.
---
## Sending the build context to Docker
```bash
Sending build context to Docker daemon 2.048 kB
```
* The build context is the `.` directory given to `docker build`.
* It is sent (as an archive) by the Docker client to the Docker daemon.
* This allows to use a remote machine to build using local files.
* Be careful (or patient) if that directory is big and your link is slow.
---
## Executing each step
```bash
Step 2/3 : RUN apt-get update
---> Running in e01b294dbffd
(...output of the RUN command...)
Removing intermediate container e01b294dbffd
---> eb8d9b561b37
```
* A container (`e01b294dbffd`) is created from the base image.
* The `RUN` command is executed in this container.
* The container is committed into an image (`eb8d9b561b37`).
* The build container (`e01b294dbffd`) is removed.
* The output of this step will be the base image for the next one.
---
## The caching system
If you run the same build again, it will be instantaneous. Why?
* After each build step, Docker takes a snapshot of the resulting image.
* Before executing a step, Docker checks if it has already built the same sequence.
* Docker uses the exact strings defined in your Dockerfile, so:
* `RUN apt-get install figlet cowsay `
is different from
`RUN apt-get install cowsay figlet`
* `RUN apt-get update` is not re-executed when the mirrors are updated
You can force a rebuild with `docker build --no-cache ...`.
---
## Running the image
The resulting image is not different from the one produced manually.
```bash
$ docker run -ti figlet
root@91f3c974c9a1:/# figlet hello
_ _ _
| |__ ___| | | ___
| '_ \ / _ \ | |/ _ \
| | | | __/ | | (_) |
|_| |_|\___|_|_|\___/
```
Yay! .emoji[🎉]
---
## Using image and viewing history
The `history` command lists all the layers composing an image.
For each layer, it shows its creation time, size, and creation command.
When an image was built with a Dockerfile, each layer corresponds to
a line of the Dockerfile.
```bash
$ docker history figlet
IMAGE CREATED CREATED BY SIZE
f9e8f1642759 About an hour ago /bin/sh -c apt-get install fi 1.627 MB
7257c37726a1 About an hour ago /bin/sh -c apt-get update 21.58 MB
07c86167cdc4 4 days ago /bin/sh -c #(nop) CMD ["/bin 0 B
4 days ago /bin/sh -c sed -i 's/^#\s*\( 1.895 kB
4 days ago /bin/sh -c echo '#!/bin/sh' 194.5 kB
4 days ago /bin/sh -c #(nop) ADD file:b 187.8 MB
```
---
## Introducing JSON syntax
Most Dockerfile arguments can be passed in two forms:
* plain string:
`RUN apt-get install figlet`
* JSON list:
`RUN ["apt-get", "install", "figlet"]`
We are going to change our Dockerfile to see how it affects the resulting image.
---
## Using JSON syntax in our Dockerfile
Let's change our Dockerfile as follows!
```dockerfile
FROM ubuntu
RUN apt-get update
RUN ["apt-get", "install", "figlet"]
```
Then build the new Dockerfile.
```bash
$ docker build -t figlet .
```
---
## JSON syntax vs string syntax
Compare the new history:
```bash
$ docker history figlet
IMAGE CREATED CREATED BY SIZE
27954bb5faaf 10 seconds ago apt-get install figlet 1.627 MB
7257c37726a1 About an hour ago /bin/sh -c apt-get update 21.58 MB
07c86167cdc4 4 days ago /bin/sh -c #(nop) CMD ["/bin 0 B
4 days ago /bin/sh -c sed -i 's/^#\s*\( 1.895 kB
4 days ago /bin/sh -c echo '#!/bin/sh' 194.5 kB
4 days ago /bin/sh -c #(nop) ADD file:b 187.8 MB
```
* JSON syntax specifies an *exact* command to execute.
* String syntax specifies a command to be wrapped within `/bin/sh -c "..."`.
---
## When to use JSON syntax and string syntax
* String syntax:
* is easier to write
* interpolates environment variables and other shell expressions
* creates an extra process (`/bin/sh -c ...`) to parse the string
* requires `/bin/sh` to exist in the container
* JSON syntax:
* is harder to write (and read!)
* passes all arguments without extra processing
* doesn't create an extra process
* doesn't require `/bin/sh` to exist in the container