All examples can be found in this repo here

What is Docker Bake?

As stated in the documentation, Bake is a wrapper around the docker build command that allows you to define how to build a Docker image.

Bake also lets you build multiple images in parallel, for multiple platforms, create multiple tags on the same image, build the same image with different build-arg values… And all this thanks to one file and a single Docker command.

It is possible to configure your Bake build using different file formats: JSON, Compose, and HCL.

In the rest of this post, all examples will be in HCL.

Scenario

Let’s imagine I need to build a Docker image with different arguments. The easiest way for me would be to write a Bash script that runs 3 docker commands, passing those arguments.

Something like this:

#!/bin/bash

set -eu

echo "START BUILDING IMAGES"

base_image_tag="base-image"

child_images_tag=("toto" "tata" "titi")

docker build -t $base_image_tag .

for child_tag in "${child_images_tag[@]}"
do
  docker build -f child.dockerfile \
  -t "$child_tag-image" \
  --build-arg CMD_ARG=$child_tag \
  --build-context baseapp="docker-image://$base_image_tag" \
  .
done

echo "DONE"

Note that we use a build context to enable inheritance between the base image and its children.

“Job done”, it’s settled, time to clock out. Yes, but here the example is obviously simpler than what you might really need.

Imagine each image takes 4 minutes to build. Here, I would wait 12 minutes in total. Whereas using Bake, I could parallelize my builds, knowing that Bake also tries to optimize layer caching between each build.

HCL equivalent of the Bash script

group "default" {
	targets = [
		"base",
		"childs"
	]
}

target "base" {
	dockerfile = "Dockerfile"
	context = "."
	tags = [
		"base-image"
	]
}

target "childs" {
	name = "image-${tag}"
	dockerfile = "child.dockerfile"
	contexts = {
		baseapp = "target:base"
	}
	tags = [
		"${tag}-image"
	]
	args = {
		CMD_ARG = "${tag}"
	}
	matrix = {
		tag = [
			"toto",
			"tata",
			"titi"
		]
	}
}

Then all I need to do is run the command, and my four images will be built.

docker buildx bake

Should I change all my Bash scripts?

Honestly, if you don’t feel the need to speed up your image builds, then you probably don’t need to waste your time migrating to Bake (after all, it’s always fun to learn).

Bake offers much more than what’s presented in this brief blog post, and I encourage you to check out the official documentation to discover: targets, groups, variables, functions, matrices, and much more.

Read the Docs!