Scratch base image for minimal containers in Docker - Mini Project: Build & Apply

Scratch Base Image for Minimal Containers

📖 Scenario: You want to create a very small Docker container that runs a simple program without any extra files or tools. Using the scratch base image helps you build the smallest possible container.This is useful when you want your container to be lightweight and secure, like sending a small package instead of a big box.

🎯 Goal: Build a Docker image using the scratch base image that runs a simple static binary program.You will create a Dockerfile step-by-step to copy your program into the container and run it.

📋 What You'll Learn

Create a Dockerfile starting from the scratch base image

Copy a static binary file named hello into the container

Set the container to run the hello program when started

Build and run the container to see the output

💡 Why This Matters

🌍 Real World

Using the scratch base image helps create very small and secure containers by including only the necessary files. This is useful for production environments where size and security matter.

💼 Career

Many DevOps and cloud engineering roles require building efficient container images. Knowing how to use scratch images is a valuable skill for optimizing container deployments.

1

Create a Dockerfile with the scratch base image

Create a file named Dockerfile and write the line FROM scratch as the first line to use the minimal base image.

Docker

# Your code here

Hint

The FROM instruction sets the base image. Use scratch for an empty base.

2

Copy the static binary into the container

Add a line to the Dockerfile to copy the file named hello from your current directory into the container's root directory /.

Docker

FROM scratch
# Your code here

Hint

Use the COPY instruction to add files into the container.

3

Set the container to run the binary

Add a line to the Dockerfile to set the default command to run the hello program when the container starts. Use the CMD instruction with the executable path /hello.

Docker

FROM scratch
COPY hello /
# Your code here

Hint

The CMD instruction defines the command to run inside the container.

4

Build and run the Docker container

Build the Docker image with the tag hello-scratch using the command docker build -t hello-scratch .. Then run the container with docker run --rm hello-scratch to see the output.

Docker

FROM scratch
COPY hello /
CMD ["/hello"]
# Your code here

Hint

Make sure your hello binary prints Hello from scratch! when run.

Practice

1. What is the main purpose of using the scratch base image in Docker?

easy

A. To automatically install dependencies for your app

B. To include a full Linux operating system inside the container

C. To create a minimal container with only your application and no extra files

D. To provide a graphical user interface for the container

Solution

1. Step 1: Understand what scratch means

   scratch is an empty base image, meaning it has no files or OS inside.

2. Step 2: Identify the purpose of minimal containers

   Using scratch lets you build very small containers by including only your app and its static binaries.

3. Final Answer:

   To create a minimal container with only your application and no extra files -> Option C

4. Quick Check:

   Minimal container = scratch base image [OK]

Hint: Scratch means empty base image for smallest containers [OK]

Common Mistakes:

• Thinking scratch includes a full OS
• Assuming scratch installs dependencies automatically
• Believing scratch provides GUI support

2. Which of the following is the correct way to specify the scratch base image in a Dockerfile?

easy

A. FROM base scratch

B. FROM scratch

C. FROM minimal/scratch

D. FROM scratch:latest

Solution

1. Step 1: Recall Dockerfile syntax for base images

   The correct syntax is FROM <image-name> without extra words.

2. Step 2: Check official usage of scratch

   scratch is a special empty image and is referenced simply as FROM scratch.

3. Final Answer:

   FROM scratch -> Option B

4. Quick Check:

   Base image syntax = FROM scratch [OK]

Hint: Use exactly 'FROM scratch' in Dockerfile [OK]

Common Mistakes:

• Adding tags like :latest to scratch
• Using extra words like 'base' or 'minimal/'
• Incorrect syntax like FROM scratch:latest

3. Given this Dockerfile snippet using scratch:

FROM scratch
COPY myapp /
CMD ["/myapp"]

What will happen if myapp is not a statically compiled binary?

medium

A. The container will fail to run because dynamic libraries are missing

B. The container will run successfully without errors

C. Docker will automatically add required libraries

D. The container will run but with degraded performance

Solution

1. Step 1: Understand scratch image limitations

   scratch has no OS or libraries, so dynamic binaries cannot find needed libraries.

2. Step 2: Analyze what happens with non-static binaries

   Non-statically compiled binaries depend on shared libraries which are missing, causing runtime failure.

3. Final Answer:

   The container will fail to run because dynamic libraries are missing -> Option A

4. Quick Check:

   Non-static binary + scratch = runtime failure [OK]

Hint: Scratch needs static binaries; dynamic ones fail [OK]

Common Mistakes:

• Assuming Docker adds missing libraries automatically
• Thinking container runs fine without libraries
• Believing performance is only affected, not failure

4. You wrote this Dockerfile:

FROM scratch
COPY app /
CMD ["/app"]

But when running the container, it fails with "exec format error". What is the most likely cause?

medium

A. The Dockerfile syntax is incorrect

B. The app file is missing execute permissions

C. The app binary is dynamically linked

D. The app binary is not compiled for the container's CPU architecture

Solution

1. Step 1: Understand "exec format error" meaning

   This error usually means the binary format does not match the CPU architecture.

2. Step 2: Check other possible causes

   Missing execute permission or dynamic linking cause different errors; syntax errors prevent build.

3. Final Answer:

   The app binary is not compiled for the container's CPU architecture -> Option D

4. Quick Check:

   Exec format error = wrong CPU architecture binary [OK]

Hint: Exec format error means binary architecture mismatch [OK]

Common Mistakes:

• Confusing exec format error with permission issues
• Assuming dynamic linking causes exec format error
• Thinking Dockerfile syntax causes this runtime error

5. You want to build a minimal Docker container using scratch that runs a Go application. Which steps should you follow to ensure the container runs correctly?

hard

A. Compile the Go app statically, use FROM scratch, copy the binary, and set CMD

B. Use FROM scratch, copy the Go source code, and run go run inside the container

C. Use FROM alpine, install Go, and compile the app inside the container

D. Compile the Go app dynamically, use FROM scratch, copy the binary, and set CMD

Solution

1. Step 1: Compile Go app statically

   Static compilation bundles all dependencies, so the binary runs without extra libraries.

2. Step 2: Use FROM scratch and copy binary

   Since scratch is empty, copy the static binary and set the command to run it.

3. Final Answer:

   Compile the Go app statically, use FROM scratch, copy the binary, and set CMD -> Option A

4. Quick Check:

   Static binary + scratch base = minimal working container [OK]

Hint: Static binary + FROM scratch + copy + CMD = minimal container [OK]

Common Mistakes:

• Trying to run source code inside scratch
• Using dynamic binaries with scratch
• Using alpine base instead of scratch for minimal size