Building Docker Images

Estimated read time 4 min read

Introduction

Dockerfiles are the building blocks for all containerized applications. In this article, we’ll take a look at how to use Dockerfiles to build custom images that run on your machine or in production.

The Docker image creation process.

The Docker image creation process is fairly straightforward. Here’s a visual breakdown:

  • Create a new directory and init it with an empty .gitignore file. This will be your project directory for this tutorial, so call it whatever you like!
  • Create a Dockerfile in the root of the project directory with instructions for Docker to follow during build time. We’ll cover these instructions in detail later on in this article!
  • From here, run `docker build -t .` In our case, we want to build our image from `./Dockerfile`, so we’d run `docker build -t heyheyheyhey/helloworld ./Dockerfile`.

Dockerfile instructions.

Dockerfile instructions are the commands that specify what to install, remove, and do to an image. They can be a single command or a series of commands.

Use the ARG command to pass values to the build process from the command line.

The ARG command is used to pass values to the build process from the command line. You can use this command to allow for build time configuration. You can set default values for an image.

With ARG, you don’t have to change your Dockerfile every time you want to switch between a development and production environment or if you want to test something in a specific environment before pushing it live.

Use multiple FROM commands to create multistage images for smaller final images.

You can use multiple FROM commands to create multistage images for smaller final images. For example, if you have a large image that you need to deploy, but only need a small part of it for your application, use multiple FROM commands and run docker build on each stage separately. This is useful if you want to separate out dependencies from your final runtime image size by building them separately and then combining them into one image later.

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Use pipe commands to keep your Dockerfile short and sweet.

  • Pipes are the way to go

In order for a container to be built, it needs to have all of its dependencies installed. If you are building multiple containers from one base image, it can get pretty annoying loading all of those dependent libraries into each image every time. This is where pipes come in handy; they allow you to pipe commands from one Dockerfile command into another (like the | operator).

Using pipes allows you keep your Dockerfiles short and sweet by creating a series of chained commands that are executed sequentially. For example:

Takeaway:

One of the most important parts of creating Docker images is to understand the syntax of a Dockerfile. It consists of a series of instructions that tell Docker how to create your image.

The syntax for each instruction is fairly straightforward, as shown below:

`FROM`: specifies which base image you want to start with (e.g., python:3 or ubuntu:latest).

`MAINTAINER`: sets contact information for the author (e.g., “John Doe”).

`RUN`: performs some action inside the container (e.g., install Python dependencies or update package lists).

Conclusion

I hope this article has helped you understand how to create Docker images. The Dockerfile is a simple yet powerful way to build your own custom container images. It’s not difficult, just remember that each line in the file starts with a command and ends with an exclamation point (!) unless it’s multiline, then it just continues until there are no more commands or text left on the same line.

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