Scratch base image for minimal containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a Scratch-based Dockerfile with a simple file copy

Consider this Dockerfile using the scratch base image:

FROM scratch
COPY hello.txt /hello.txt
CMD ["/hello.txt"]

What will happen when you run this container?

Docker

FROM scratch
COPY hello.txt /hello.txt
CMD ["/hello.txt"]

A The container will fail to start because /hello.txt is not executable

B The container will start and print the contents of hello.txt to the console

C The container will start and run an empty shell

D The container will fail to build because scratch cannot copy files

💡 Hint

Scratch images have no shell or runtime environment.

The scratch image is empty and has no shell or runtime. Copying a text file and trying to run it as a command will fail because the file is not executable.

🧠 Conceptual

intermediate

Why use the scratch base image in Docker?

Which of the following is the main reason to use the scratch base image in Docker containers?

A To create the smallest possible container image with only your application and its dependencies

B To automatically include debugging tools in the container

C To enable running containers with a graphical user interface

D To have a full Linux environment with all standard utilities

💡 Hint

Think about container size and minimalism.

The scratch image is empty and used to build minimal containers containing only the application and its required files, reducing size and attack surface.

❓ Troubleshoot

advanced

Troubleshooting a failing scratch container with a Go binary

You built a Go application statically linked and created this Dockerfile:

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

The container fails to start with 'exec format error'. What is the most likely cause?

Docker

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

A The CMD syntax is incorrect and should use shell form

B The Go binary was compiled for a different CPU architecture than the container host

C The scratch image does not support running Go binaries

D The app file is missing execute permissions

💡 Hint

Exec format errors usually relate to binary compatibility.

An exec format error means the binary is not compatible with the CPU architecture of the host. Scratch image runs the binary directly, so architecture must match.

🔀 Workflow

advanced

Correct workflow to build a minimal scratch container for a static binary

Which sequence of commands correctly builds and runs a minimal scratch container for a statically compiled binary named 'myapp'?

A 1. docker build -t myapp . 2. docker exec myapp /myapp

B 1. docker run -it scratch 2. copy myapp inside container 3. run /myapp

C 1. docker build -t myapp . 2. docker run -it --rm myapp /bin/sh

D 1. docker build -t myapp . 2. docker run myapp

💡 Hint

Think about building an image and then running it.

You build the image with docker build and then run it with docker run. Scratch images have no shell, so running /bin/sh or exec inside won't work.

✅ Best Practice

expert

Best practice for adding dependencies to a scratch-based container

You want to create a scratch-based container that runs a Python script. Which approach is best to include Python and your script in the container?

A Install Python inside the scratch image using apt-get

B Directly copy the Python script into the scratch image and run it with CMD

C Use a multi-stage build: first stage with a full Python image to install dependencies and copy the Python interpreter and libraries into the scratch image

D Use the scratch image and run the script with 'python script.py' assuming Python is available

💡 Hint

Scratch images are empty and cannot install packages by themselves.

Scratch images are empty. To run Python, you must copy the Python interpreter and libraries from a full image using multi-stage builds.

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