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.