Understanding Dockerfiles: Structure and Common Instructions

--- title: Writing a Dockerfile keywords: concepts, build, images, container, docker desktop description: This concept page will teach you how to create image using Dockerfile. summary: | Mastering Dockerfile practices is vital for leveraging container technology effectively, enhancing application reliability and supporting DevOps and CI/CD methodologies. In this guide, you’ll learn how to write a Dockerfile, how to define a base image and setup instructions, including software installation and copying necessary files. weight: 2 aliases: - /guides/docker-concepts/building-images/writing-a-dockerfile/ --- {{< youtube-embed Jx8zoIhiP4c >}} ## Explanation A Dockerfile is a text-based document that's used to create a container image. It provides instructions to the image builder on the commands to run, files to copy, startup command, and more. As an example, the following Dockerfile would produce a ready-to-run Python application: ```dockerfile FROM python:3.12 WORKDIR /usr/local/app # Install the application dependencies COPY requirements.txt ./ RUN pip install --no-cache-dir -r requirements.txt # Copy in the source code COPY src ./src EXPOSE 5000 # Setup an app user so the container doesn't run as the root user RUN useradd app USER app CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8080"] ``` ### Common instructions Some of the most common instructions in a `Dockerfile` include: - `FROM <image>` - this specifies the base image that the build will extend. - `WORKDIR <path>` - this instruction specifies the "working directory" or the path in the image where files will be copied and commands will be executed. - `COPY <host-path> <image-path>` - this instruction tells the builder to copy files from the host and put them into the container image. - `RUN <command>` - this instruction tells the builder to run the specified command. - `ENV <name> <value>` - this instruction sets an environment variable that a running container will use. - `EXPOSE <port-number>` - this instruction sets configuration on the image that indicates a port the image would like to expose. - `USER <user-or-uid>` - this instruction sets the default user for all subsequent instructions. - `CMD ["<command>", "<arg1>"]` - this instruction sets the default command a container using this image will run. To read through all of the instructions or go into greater detail, check out the [Dockerfile reference](https://docs.docker.com/engine/reference/builder/). ## Try it out Just as you saw with the previous example, a Dockerfile typically follows these steps: 1. Determine your base image 2. Install application dependencies 3. Copy in any relevant source code and/or binaries 4. Configure the final image In this quick hands-on guide, you'll write a Dockerfile that builds a simple Node.js application. If you're not familiar with JavaScript-based applications, don't worry. It isn't necessary for following along with this guide. ### Set up [Download this ZIP file](https://github.com/docker/getting-started-todo-app/raw/build-image-from-scratch/app.zip) and extract the contents into a directory on your machine.

A Dockerfile is a text document containing instructions to build a Docker image, including specifying a base image, copying files, running commands, setting environment variables, exposing ports, and defining the default command. Common instructions include FROM, WORKDIR, COPY, RUN, ENV, EXPOSE, USER, and CMD. The process typically involves determining a base image, installing dependencies, copying source code, and configuring the final image.