Building images with docker build - Deep Dive

Overview - Building images with docker build

What is it?

Building images with docker build means creating a snapshot of an application and its environment using instructions in a Dockerfile. This snapshot, called an image, can be used to run containers anywhere. The docker build command reads the Dockerfile and assembles the image step-by-step. It makes packaging software easy and consistent.

Why it matters

Without docker build, developers would struggle to package applications with all their dependencies in a reliable way. This would cause software to work on one machine but fail on another. Docker build solves this by creating portable images that run the same everywhere, saving time and avoiding bugs caused by environment differences.

Where it fits

Before learning docker build, you should understand basic Docker concepts like containers and images. After mastering docker build, you can learn about image optimization, multi-stage builds, and deploying containers using orchestration tools like Kubernetes.

Mental Model

Core Idea

Docker build reads a recipe (Dockerfile) and cooks a ready-to-use meal (image) that can be served (run) anywhere.

Think of it like...

Imagine you have a recipe book (Dockerfile) that tells you how to make a cake step-by-step. Using the recipe, you bake the cake (build the image). Once baked, you can share the cake with friends anywhere, and it tastes the same every time.

Docker build process:

┌─────────────┐
│ Dockerfile  │
└──────┬──────┘
       │ Reads instructions
       ▼
┌─────────────┐
│ Build Steps │
│ (RUN, COPY) │
└──────┬──────┘
       │ Executes each step
       ▼
┌─────────────┐
│   Image     │
│ (Snapshot)  │
└─────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding Docker Images and Containers

Concept: Learn what Docker images and containers are and how they relate.

A Docker image is like a blueprint or snapshot of an application and its environment. A container is a running instance of that image. You can think of an image as a saved game, and a container as playing that game live. Images are built once and can be run many times as containers.

Result

You understand that images are static files and containers are active processes created from images.

Knowing the difference between images and containers is essential because building images is about creating the blueprint, not running the app.

2

Foundation

What is a Dockerfile?

Concept: Dockerfile is a text file with instructions to build an image.

A Dockerfile contains commands like FROM (base image), RUN (execute commands), COPY (add files), and CMD (default command). Docker build reads this file line by line to create the image. It’s like a recipe that tells Docker how to prepare the image.

Result

You can identify Dockerfile instructions and understand their purpose in image creation.

Recognizing Dockerfile as the source of truth for image building helps you control exactly what goes into your image.

3

Intermediate

Using docker build Command Basics

🤔Before reading on: do you think docker build needs a Dockerfile in the current folder or can it build without one? Commit to your answer.

Concept: Learn how to run docker build with basic options to create an image from a Dockerfile.

The basic command is: docker build -t imagename:tag . - The dot (.) means build context is current folder. - -t tags the image with a name and optional version. Docker reads the Dockerfile in the context folder and builds the image step-by-step.

Result

An image named 'imagename:tag' is created and listed in docker images.

Understanding the build context and tagging is key to managing images and avoiding confusion.

4

Intermediate

How Build Context Affects Image Creation

🤔Before reading on: do you think docker build sends the entire computer files or just the folder you specify? Commit to your answer.

Concept: Build context is the set of files sent to Docker daemon during build; it affects what files can be copied into the image.

When you run docker build with a folder, Docker sends all files in that folder (and subfolders) to the daemon. COPY and ADD commands can only access files inside this context. Sending too many files slows build and can leak secrets if not careful.

Result

You learn to organize your project so only necessary files are in the build context.

Knowing build context prevents accidental inclusion of sensitive or large files and speeds up builds.

5

Intermediate

Layer Caching Speeds Up Builds

🤔Before reading on: do you think docker build always rebuilds every step or can it reuse previous work? Commit to your answer.

Concept: Docker build caches each step as a layer and reuses unchanged layers to speed up rebuilds.

Each Dockerfile instruction creates a layer. If a step and its inputs haven't changed, Docker reuses the cached layer instead of running it again. This makes builds faster, especially during development. But changing early steps invalidates cache for later steps.

Result

Build times improve significantly when cache is used properly.

Understanding caching helps you write Dockerfiles that build faster by ordering instructions wisely.

6

Advanced

Multi-Stage Builds for Smaller Images

🤔Before reading on: do you think you can use multiple FROM instructions in one Dockerfile? Commit to your answer.

Concept: Multi-stage builds let you use multiple FROM statements to separate build environment from final image, reducing size.

You can have several FROM lines in a Dockerfile. The first stages build or compile code with all tools. The last stage copies only the needed artifacts into a clean base image. This avoids shipping build tools and reduces image size.

Result

Final images are smaller and more secure, containing only runtime essentials.

Knowing multi-stage builds is crucial for production-ready images that are efficient and secure.

7

Expert

How Docker Build Handles Secrets and Sensitive Data

🤔Before reading on: do you think secrets added during build remain in the final image? Commit to your answer.

Concept: Docker build can leak secrets if not handled carefully because build steps are cached and layers saved.

If you add secrets (like passwords) in RUN commands or COPY files, they become part of image layers and can be extracted later. Docker BuildKit offers secret management to pass secrets only during build without saving them in layers. Using BuildKit requires enabling it and using special syntax.

Result

You learn to protect sensitive data during image builds and avoid accidental leaks.

Understanding secret handling prevents serious security risks in container images.

Under the Hood

Docker build runs on the Docker daemon, which reads the Dockerfile instructions sequentially. Each instruction creates a new read-only layer on top of the previous image. These layers are cached and stored separately. When building, Docker sends the build context (files) to the daemon. The daemon executes commands like RUN inside temporary containers, commits the result as a new layer, and moves to the next instruction. The final image is a stack of these layers.

Why designed this way?

This layered design allows efficient storage and reuse of common parts between images. Caching layers speeds up rebuilds by avoiding repeated work. Separating build instructions into layers also helps with versioning and sharing. Alternatives like building everything in one step would be slower and less flexible.

Docker build internal flow:

┌─────────────┐
│ Dockerfile  │
└──────┬──────┘
       │
       ▼
┌─────────────┐
│ Build Context│
│ (Files)     │
└──────┬──────┘
       │
       ▼
┌─────────────────────┐
│ Docker Daemon        │
│ ┌─────────────────┐ │
│ │ Executes Steps  │ │
│ │ (RUN, COPY...)  │ │
│ └──────┬──────────┘ │
│        ▼            │
│ ┌───────────────┐   │
│ │ Creates Layers │   │
│ └───────────────┘   │
└─────────┬───────────┘
          ▼
    ┌─────────────┐
    │ Final Image │
    └─────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does docker build always rebuild every step even if nothing changed? Commit yes or no.

Common Belief: Docker build always runs every instruction from scratch every time.

Reality: Docker build caches layers and reuses them if the instruction and context haven't changed.

Why it matters: Believing this causes frustration over slow builds and missed opportunities to optimize Dockerfiles.

Quick: Can you copy files from anywhere on your computer during docker build? Commit yes or no.

Common Belief: You can COPY any file from your computer into the image during build.

Reality: You can only COPY files inside the build context folder sent to Docker daemon.

Why it matters: Trying to copy files outside the context causes build errors and confusion.

Quick: If you add a secret in a RUN command, does it stay hidden in the final image? Commit yes or no.

Common Belief: Secrets used during build are not saved in the final image layers.

Reality: Secrets added in RUN or COPY commands become part of image layers and can be extracted later.

Why it matters: This misconception leads to serious security leaks in container images.

Quick: Can you have multiple FROM instructions in a Dockerfile? Commit yes or no.

Common Belief: A Dockerfile can only have one FROM instruction.

Reality: Dockerfiles can have multiple FROM instructions to create multi-stage builds.

Why it matters: Not knowing this limits image optimization and results in larger, less secure images.

Expert Zone

1

Docker build cache is invalidated not only by changes in instructions but also by changes in files referenced by COPY or ADD, which can cause unexpected rebuilds.

2

The order of instructions in Dockerfile greatly affects build speed and image size; placing frequently changing commands later preserves cache for earlier steps.

3

BuildKit, a modern builder backend, supports advanced features like secret management and parallel builds, but requires explicit enabling and syntax changes.

When NOT to use

Docker build is not suitable for building images that require dynamic runtime configuration or secrets that must never be stored in images. In such cases, use runtime environment variables, external secret managers, or orchestration tools that inject secrets at container start.

Production Patterns

In production, multi-stage builds are standard to produce minimal images. CI/CD pipelines automate docker build with caching and tagging strategies. BuildKit is used for secure secret handling. Images are scanned for vulnerabilities post-build before deployment.

Connections

Continuous Integration (CI)

Docker build is often integrated into CI pipelines to automate image creation on code changes.

Understanding docker build helps grasp how automated testing and deployment pipelines package and deliver software reliably.

Version Control Systems (Git)

Docker build context usually includes source code managed by Git, linking code changes to image builds.

Knowing how build context works clarifies why .dockerignore files are like .gitignore, controlling what files are included.

Manufacturing Assembly Lines

Docker build layers resemble assembly line stages where each step adds parts to the product.

Seeing build steps as assembly stages helps understand caching and why changing early steps affects the whole build.

Common Pitfalls

#1 Including unnecessary files in build context slows build and leaks sensitive data.

Wrong approach:

docker build -t myapp . # with large .git, node_modules, or secret files included

Correct approach:

Add a .dockerignore file to exclude unnecessary files, then run docker build -t myapp .

Root cause:

Not using .dockerignore or misunderstanding build context scope causes large, slow builds and security risks.

#2 Placing frequently changing commands early in Dockerfile causes cache invalidation and slow rebuilds.

Wrong approach:

FROM ubuntu
RUN apt-get update && apt-get install -y curl
COPY . /app
RUN make build

Correct approach:

FROM ubuntu
RUN apt-get update && apt-get install -y curl
COPY . /app
RUN make build

Root cause:

Misunderstanding that cache depends on instruction order leads to inefficient builds.

#3 Adding secrets directly in RUN commands exposes them in image layers.

Wrong approach:

RUN echo 'mysecretpassword' > /secret.txt

Correct approach:

# Use BuildKit secrets feature or pass secrets at runtime instead

Root cause:

Lack of awareness about how build layers store all command outputs causes security leaks.

Key Takeaways

Docker build creates images by reading a Dockerfile and executing instructions step-by-step to produce a portable snapshot.

Build context defines which files are available during build; managing it carefully improves speed and security.

Docker caches each build step as a layer, speeding up rebuilds when instructions or files don't change.

Multi-stage builds allow creating small, secure images by separating build and runtime environments.

Handling secrets during build requires special care to avoid leaking sensitive data in image layers.