Table of Contents
Overview of a Dockerfile’s Shape
A Dockerfile is a plain text file that describes how to build a Docker image. Even before you learn every instruction in detail, it is very useful to recognize the typical shape that most Dockerfiles follow. This chapter focuses on that overall structure, not on all the individual commands, which you will explore elsewhere.
Every Dockerfile is read from top to bottom. Docker builds the image step by step from the first instruction to the last one. Because of this, the order of lines in a Dockerfile is not just cosmetic, it affects build speed, caching, and sometimes the behavior of the final image.
A Dockerfile is processed from top to bottom, and each instruction creates a new image layer. The order of instructions affects caching and the final image content.
A basic Dockerfile will usually include a base image, some configuration of the filesystem and environment, installation of dependencies, adding your application code, and a default command for containers created from the image.
The Mandatory Starting Point
Every useful Dockerfile begins by declaring which existing image it will build upon. This is the base image. In a simple Dockerfile, this is typically the very first instruction. Without a base image, Docker would not know which operating system files or runtime to provide.
Later chapters will cover the specific FROM instruction, but for structure it is enough to understand that the first visible building block is always a base to stand on. This establishes what the rest of the file can assume, such as available package managers or tools.
Grouping Related Steps
Although Docker does not require logical grouping, human readers do. A clear structure makes your Dockerfile easier to maintain and to debug. A common pattern in basic Dockerfiles is to organize instructions into clear sections that follow a natural story.
Right after the base image, you typically see a block that prepares the environment. This might include setting configuration values, creating directories for your app, or defining useful metadata. Then there is usually a focused area where dependencies are installed. After that, your own application files are copied in and possibly built. Finally, at the bottom, you set what should happen when a container starts.
This top to bottom structure often looks like a narrative: choose a base, prepare it, bring in dependencies, add the application, define how to run it.
Filesystem Layout and Application Code
One very visible part of a Dockerfile’s structure concerns where inside the image your application will live. A basic Dockerfile usually decides on a working directory and then places code and related files into that directory.
The decision of directory comes before the steps that copy files. In simple Dockerfiles, this order helps keep all application files under one path and makes later commands simpler, because many of them can assume the same current directory. Once the directory is established, the Dockerfile copies the relevant content of your project into the image. This pairing of “set a working directory” followed by “copy application files” is a structural pattern you will see often.
Separating System Setup from Application Setup
Another important feature of basic Dockerfile structure is the separation between system level preparation and application level preparation. System level preparation concerns actions that affect the whole image environment, such as installing system packages, tools, or runtimes. Application level preparation concerns steps specific to your code, such as installing application libraries or building assets.
In a simple Dockerfile, you will typically see system setup instructions closer to the top. They rarely change once you pick your stack. Application setup, like installing your project’s dependencies, appears nearer to where you copy in your code. This arrangement helps Docker reuse cached layers when only your application code changes and the underlying system remains the same.
Keep stable system setup steps higher in the Dockerfile and frequently changing application steps lower. This structure takes better advantage of Docker’s build cache.
Even without knowing the exact commands, you can usually tell which part of the file is concerned with the operating environment and which part is focused on your own project.
Defining Container Behavior at the End
The bottom section of a basic Dockerfile usually answers one crucial question. Once an image is built, what will a container based on that image do by default?
The final lines tend to define the container’s starting behavior. This might be running a server, a script, or some other process that is central to your application. In many Dockerfiles, this is the last visible block and it is deliberately placed there because it depends on everything above. The application code, dependencies, and configuration must already be in place before the container can run anything.
Because Docker reads from top to bottom, keeping the startup definition at the end ensures that any change in how your application runs does not unexpectedly interfere with earlier structural parts of the Dockerfile.
Commenting and Readability
While not mandatory, comments are an important structural element in real Dockerfiles. In a basic layout, comments act as section headers inside the file. For example, you may see a comment above the block that installs system dependencies, another before application specific steps, and another near the bottom that marks the part that defines how the container starts.
This use of comments helps new readers quickly understand the purpose of each region of the file. It turns a long list of instructions into a document with recognizable sections, which matches the conceptual structure you learned in this chapter.
Putting the Structure Together
If you read an unfamiliar Dockerfile from top to bottom and try to identify its sections, you will often be able to map it to the basic structure described here. First, you see the base image declaration. Then, a group of environment and system setup steps. Next, the working directory and application files. After that, the application specific preparation. Finally, the definition of what a container should do when it starts.
Understanding this shape is a foundation. Once you recognize these structural building blocks, the specific instructions you learn in other chapters will be much easier to place and reason about.