Docker Build - Shell/Bash

Docker is a platform that allows developers to easily create, deploy, and run applications in containers. One of the key components of Docker is the docker build command, which allows developers to build container images from a Dockerfile.

What is a Dockerfile?

A Dockerfile is a text file that contains a set of instructions for building a container image. These instructions include things like which base image to use, what packages to install, and how to configure the container. Dockerfiles can be version-controlled and shared with other developers, making it easy to reproduce and deploy applications across different environments.

Building an image with docker build

To build an image from a Dockerfile, you can use the docker build command. Here's an example:

docker build -t my-image:latest .

This command tells Docker to build an image with the tag my-image:latest, using the Dockerfile in the current directory (.).

The docker build command will read the Dockerfile and execute each instruction in order. If any step fails, the build will stop and you'll get an error message.

Understanding the Docker build context

When you run docker build, Docker creates a build context that includes all the files in the directory where the Dockerfile is located, as well as any subdirectories. This context is sent to the Docker daemon, which uses it to build the container image.

It's important to understand the build context, as it can have a big impact on the size and speed of your container image. For example, if you have large files in your build context that aren't needed in the final image, you can exclude them using a .dockerignore file.

Using build arguments

Sometimes, you may want to pass build-time variables to your Dockerfile, such as environment-specific settings or secrets. You can do this using build arguments.

Here's an example:

docker build --build-arg MYVAR=foo -t my-image:latest .

This command sets the MYVAR build argument to foo, which can then be accessed in the Dockerfile as ${MYVAR}.

Conclusion

The docker build command is a powerful tool for creating container images from Dockerfiles. By understanding the build context, using build arguments, and following best practices, you can create efficient and reproducible container images that can be easily deployed and shared.