A Dockerfile is a text document that contains the instructions for building a Docker image. To write a Dockerfile, create a new file called “Dockerfile” (without any extension) and enter the instructions in the file.
Here is a simple example of a Dockerfile that creates an image based on the Ubuntu operating system and installs the Apache web server:
# Use the Ubuntu base image FROM ubuntu:20.04 # Update the package manager and install Apache RUN apt-get update && apt-get install -y apache2 # Expose the default Apache port EXPOSE 80 # Run Apache in the foreground CMD ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]
In this example, the
FROM instruction specifies the base image to use, the
RUN instruction installs the Apache web server, the
EXPOSE instruction tells Docker to expose port 80, and the
CMD instruction tells Docker to run Apache in the foreground.
To build a Docker image from this Dockerfile, use the
docker build command. For example:
docker build -t my-apache-image .
This will build an image with the name “my-apache-image” based on the instructions in the Dockerfile.
Best Practices for Dockerfile
Here are a few best practices for writing Dockerfiles:
- Use a minimal base image: Start your Dockerfile with a minimal base image, such as a Linux distribution’s official “slim” or “alpine” image. This will help keep your image small and reduce the attack surface.
- Use multi-stage builds: Use multi-stage builds to separate the build environment from the runtime environment. This will allow you to keep the build tools and dependencies out of the final image, reducing its size and improving its security.
- Use the
.dockerignorefile: Use a
.dockerignorefile to exclude files and directories that don’t need to be included in the image. This will help keep your image small and reduce the build time.
- Use the
COPYinstruction instead of
ADD: Use the
COPYinstruction instead of the
ADDinstruction to copy files and directories into the image. Unlike
COPYdoes not automatically extract archives or download files from the internet, which can help prevent security issues.
RUNto install packages and dependencies: Use the
RUNinstruction to install packages and dependencies in your Dockerfile. This will help ensure that the packages and dependencies are installed in a consistent and reproducible way.
- Use labels: Use labels to add metadata to your Docker images, such as the maintainer’s name and contact information, the version number, and the application’s name and description. This will make it easier to manage and organize your images.
Contact us to apply best practices to your Dockerfiles!