What is a Dockerfile?

The recipe for an image

In the last chapter we said an image is like a recipe. A Dockerfile is where you write that recipe. It's a plain text file, named exactly Dockerfile (no extension), containing a list of instructions. Docker reads them top to bottom and builds an image from them.

Each instruction is written in capital letters, followed by its arguments. Here's a tiny example so you can see the shape - don't worry about the details yet:

FROM alpine
CMD ["echo", "Hello from my own image"]

Reading it in plain English:

• FROM alpine - start from the small alpine Linux image as a base.
• CMD [...] - when a container starts from this image, run this command.

Why write a Dockerfile?

A Dockerfile lets you describe your app's environment as code. Instead of telling a teammate "install PHP 8.4, then this extension, then copy these files, then run this", you write it once in a Dockerfile. Anyone can then build the exact same image with a single command.

That gives you three big wins:

• Repeatable - the same Dockerfile always produces the same setup.
• Shareable - commit it to your project so the whole team uses it.
• Version-controlled - it lives in git next to your code, so changes are tracked.

The most common instructions

You'll learn these one at a time in the next lessons, but here's the list so the words look familiar:

• FROM - the base image to build on.
• WORKDIR - the working directory inside the image.
• COPY - copy files from your computer into the image.
• RUN - run a command while building the image.
• CMD / ENTRYPOINT - what to run when a container starts.

Let's write a real Dockerfile in the next lesson.

One detail that saves confusion later

The file must be named exactly Dockerfile - capital D, no extension. Beginners often save it as dockerfile.txt or Dockerfile.md and then wonder why docker build can't find it. If your build complains it can't locate the Dockerfile, check the filename first. (You can use a different name, but then you have to point Docker at it explicitly, which we won't need in this course.)

FAQ

What is a Dockerfile used for?

It's the recipe Docker follows to build an image. You list instructions - the base image, files to copy, commands to run - and docker build turns them into a reusable image you can run anywhere.

Is a Dockerfile the same as docker-compose?

No. A Dockerfile describes how to build one image. Docker Compose (a later chapter) describes how to run several containers together. They're complementary, not the same tool.

Does the Dockerfile need a file extension?

No - it's named Dockerfile with no extension. That exact name is what docker build looks for by default.