Dockerfile intro

A Dockerfile is a text file that contains instructions for building an image that defines the runtime environment for each container launched using that image.

In the upcoming examples, you will be introduced to the following instructions:


The FROM instruction is used to declare the base image for the new image. This makes it possible to compose images from other existing images that daisy chain the dependencies your app might require, such as:

  • A language runtime, such as Node.js, that depends on another base image like
  • A Linux distribution, such as Ubuntu.


Normally what distinguishes your image from its base image are the files that you add to it. You might be building an image intended to be used as a more specialized base image for yet another image, or you might be building an image for a program that you’ve created and want to deploy.

Either way, you can will normally use the COPY instruction to copy files from your host system into the image.

🏆 Best practice

There is also an ADD instruction that is similar, but we won’t be covering it since using COPY is preferred. See this best practices section to learn more.


These instructions are used to specify the command that should be run when a container is launched, along with any default options. There are differences between the effect of using the two instructions, and there are reasons why it is often ideal to use them together.

The helloworld example will introduce ENTRYPOINT, and the time example will expand and clarify both the ENTRYPOINT and CMD instructions when used separately and together.

🤓 Nerd note

Keep in mind, of course, that if you’re building an image that you intend to be used as a specialized base image for other images, it’s perfectly normal not to set either an ENTRYPOINT or a CMD.