lesson-6

Dockerfile

In the earlier lesson, we

• started a base container with ubuntu:latest image,
• installed a new tool, curl,
• hit a URL with curl and saved it's data in a file,
• copied a file from host to a container,
• finally, saved it as a new image.

Let's learn to do the same in a much more simpler, declarative way, using Dockerfile.

Before that, create a new directory on your host machine and work inside it. You'll find later why.

$ mkdir ~/docker-demo && cd ~/docker-demo # Create and move to a directory
$ echo "hello-world" > myfile.txt         # Create a file that we'll copy later
$ touch Dockerfile                        # Create an empty file where we'll do our work

Here's the Dockerfile for the stuff we did in previous lesson.

FROM ubuntu:latest

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

WORKDIR /mydata

RUN curl https://aliyousuf.com > index.html

ADD myfile.txt hello.txt

Let's break it down.

Line 1: FROM <image name>

Every Docker image has a base image. Or the image from which it spring off. Hence FROM is necessary in every Dockerfile.

Our image will be based on ubuntu:latest image.

Line 2 & 4: RUN <command>

This is similar to docker exec -it <command>.

We're using it twice. Once to install curl and second to use curl to download a file.

Line 3: WORKDIR <path>

WORKDIR changes the current working directory. If the directory doesn't exists, it creates the directory and cd into it.

Like in our case, we haven't created /mydata yet. But this command will create it for us.

You might wonder, why not use RUN mkdir /mydata && cd /mydata?

Every command (RUN or ADD, for example) uses the last value set by WORKDIR as the current working directory. So it saves you from adding cd /mydata to every other command.

Line 5: ADD <src path> <dst path>

ADD performs the task of docker cp command. But in this case, we're using a relative paths, not absolute.

For <dst path>, you might have guessed it. Its absolute path becomes <last workdir path>/<dst path>.

For <src path>, it's a bit complicated. The absolute path it solves to is <context path>/<src path>.

But what is <context path>? Let's see it in the next heading.

Building an image

Now you've got your Dockerfile ready, but how do you create an image using it?

$ docker build --file <path to Dockerfile> --tag <new image name> <context path>

So your command should look like this (with shorter flags):

$ docker build -f Dockerfile -t my-second-image:latest .

Yup, there's a period in the end. That's the context path, we've set it to use the current directory.

Context path is used by Dockerfile to resolve relative paths like in ADD command. That's how ADD command finds out where is the myfile.txt located.

Now, let's be more useful with our containers in the next lesson.