Naming Your Docker Images During Build

Docker provides a powerful way to package and distribute applications, but often you’ll want to specify the name and tag of your image during the build process itself. While the Dockerfile primarily defines how to build the image, it doesn’t directly dictate the final image name. This tutorial explains how to achieve this, and explores common approaches for streamlining your Docker builds.

Understanding Image Naming

When you use the docker build command, you typically provide a tag (name:tag) to identify your image. For example:

docker build -t myapp:latest .

This command builds an image from the Dockerfile in the current directory and tags it as myapp:latest. Without a tag, Docker assigns an anonymous ID to the image, making it harder to manage and share. The core challenge is that the Dockerfile itself doesn’t have a built-in instruction to set the image name.

Using docker build -t

The primary and most straightforward method is to leverage the -t flag with the docker build command. This is the recommended approach for most use cases.

docker build -t your-username/your-image-name:tag .

Replace your-username, your-image-name, and tag with your desired values. The tag is optional; if omitted, latest is assumed.

This method is simple, clear, and doesn’t require any changes to your Dockerfile. It directly addresses the problem by providing the image name during the build invocation.

Automating with Build Scripts (Makefile)

For more complex projects, you might want to automate the build process. A Makefile can be a helpful tool. Create a file named Makefile in your project directory with content similar to the following:

.PHONY: all
all: build

build:
	docker build -t your-username/your-image-name:tag .

Now, instead of running the docker build command directly, you can simply run make. This will execute the build target, which in turn runs the docker build command with your specified tag. This keeps your build command consistent and repeatable.

Leveraging docker-compose

docker-compose is a tool for defining and running multi-container Docker applications. It can also be used to build images with specified tags. Create a docker-compose.yml file:

version: "3"
services:
  your_service:
    build: .
    image: your-username/your-image-name:tag

Then, run docker-compose build to build your image with the specified tag. This is particularly useful when you’re working with more complex projects involving multiple services.

Advanced: Using buildah

For greater flexibility and control, consider using buildah. buildah is a tool specifically designed for building OCI (Open Container Initiative) images. It allows you to build images without requiring a Docker daemon and offers more granular control over the build process.

Here’s a basic example:

buildah from alpine:3
buildah mount
buildah copy . /
buildah config --author "Your Name"
buildah commit --tag your-username/your-image-name:tag

This script creates a new container from the alpine:3 image, mounts it, copies your project files into it, sets the author, and commits the changes to a new image with the specified tag. buildah provides a powerful alternative to docker build and can be a valuable tool for advanced users.

Choosing the Right Approach

• For simple projects and one-off builds, using docker build -t is the easiest and most straightforward solution.
• For projects that require automation or repeatability, a Makefile is a good choice.
• When working with multi-container applications, docker-compose provides a convenient way to build and manage your images.
• For advanced users who need more control over the build process, buildah offers a powerful alternative.