Distroless images concept in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a distroless image in Docker?

A distroless image is a minimal Docker image that contains only the application and its runtime dependencies, without any operating system shell or package manager.

beginner

Why are distroless images considered more secure?

Because they exclude shells and package managers, reducing the attack surface and limiting what an attacker can do if they gain access to the container.

beginner

How do distroless images affect the size of Docker containers?

Distroless images are smaller because they remove unnecessary OS components, which helps reduce the container size and speeds up deployment.

beginner

Can you run shell commands inside a distroless container?

No, distroless images do not include a shell, so you cannot run shell commands inside the container.

beginner

What is a common use case for distroless images?

They are commonly used in production environments where security and small image size are important, especially for microservices and cloud-native applications.

What is missing from a distroless Docker image compared to a traditional base image?

A Operating system shell and package manager

B Application runtime

C Network support

D Container runtime

Distroless images remove the OS shell and package manager to reduce size and improve security.

Why might you choose a distroless image for your container?

A To include development tools

B To have a full Linux environment inside the container

C To enable easy debugging with shell access

D To reduce image size and improve security

Distroless images focus on minimal size and security, not on providing a full OS or debugging tools.

Which of the following is true about distroless images?

A They include a bash shell by default

B They are larger than traditional images

C They contain only the app and its runtime dependencies

D They are only used for development

Distroless images contain only the application and necessary runtime dependencies.

If you need to debug inside a container, what is a limitation of distroless images?

A They lack a shell to run commands

B They cannot run applications

C They do not support networking

D They are read-only

Distroless images do not include a shell, so you cannot run interactive commands inside.

Which scenario best fits using a distroless image?

A Developing and testing new features interactively

B Running a secure, lightweight production service

C Building a full Linux desktop environment

D Running containers that require package installation at runtime

Distroless images are ideal for secure, lightweight production deployments.

Explain what a distroless image is and why it is useful in Docker container deployments.

Think about what is removed from the image and the benefits it brings.

Describe the trade-offs when using distroless images compared to traditional Docker base images.

Consider what you gain and what you lose.

Practice

1. What is the main advantage of using distroless images in Docker containers?

easy

A. They keep containers small and secure by including only necessary files

B. They include a full Linux distribution for easier debugging

C. They come with built-in shells and package managers

D. They automatically update the application inside the container

Solution

1. Step 1: Understand what distroless images contain

   Distroless images include only the application and its runtime dependencies, excluding shells and package managers.

2. Step 2: Identify the benefit of minimal content

   By including only what is needed, the image size is smaller and the attack surface is reduced, improving security.

3. Final Answer:

   They keep containers small and secure by including only necessary files -> Option A

4. Quick Check:

   Minimal content = Small and secure [OK]

Hint: Distroless means minimal, no shells or extras included [OK]

Common Mistakes:

• Thinking distroless images have shells for debugging
• Assuming they include full Linux distributions
• Believing they auto-update applications

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

easy

A. FROM ubuntu:latest

B. FROM alpine:3.15

C. FROM gcr.io/distroless/base

D. FROM node:distroless

Solution

1. Step 1: Identify official distroless image naming

   Distroless images are hosted at gcr.io/distroless with names like 'base', 'static', or language-specific tags.

2. Step 2: Compare options to distroless naming

   Only 'gcr.io/distroless/base' matches the official distroless image naming convention.

3. Final Answer:

   FROM gcr.io/distroless/base -> Option C

4. Quick Check:

   Official distroless images start with gcr.io/distroless [OK]

Hint: Distroless images start with gcr.io/distroless prefix [OK]

Common Mistakes:

• Using common Linux distros like ubuntu or alpine as distroless
• Assuming 'node:distroless' is a valid tag
• Confusing distroless with minimal Linux images

3. Given this Dockerfile snippet using a distroless image:

FROM gcr.io/distroless/base
COPY app /app
CMD ["/app"]

What will happen if you try to run docker run and then execute sh inside the container?

medium

A. You will get a shell prompt because distroless images include shells

B. You will get an error saying 'sh: not found' because no shell is included

C. The container will crash immediately on start

D. The container will open a shell automatically

Solution

1. Step 1: Understand distroless image contents

   Distroless images do not include shells or package managers, only the app and runtime.

2. Step 2: Predict shell availability inside container

   Trying to run 'sh' will fail because the shell binary is missing, causing 'sh: not found' error.

3. Final Answer:

   You will get an error saying 'sh: not found' because no shell is included -> Option B

4. Quick Check:

   No shell in distroless = 'sh: not found' error [OK]

Hint: Distroless images have no shell, so 'sh' command fails [OK]

Common Mistakes:

• Assuming shells exist for debugging
• Expecting container to crash on start
• Thinking shell opens automatically

4. You wrote this Dockerfile using a distroless image:

FROM gcr.io/distroless/base
COPY app /app
CMD ["/app"]

But when you run the container, it exits immediately with no logs. What is the most likely cause?

medium

A. The CMD syntax is incorrect for distroless images

B. The distroless image requires a shell to run commands

C. The container needs a package manager to start

D. The app binary is missing executable permissions

Solution

1. Step 1: Check distroless image requirements

   Distroless images run the app binary directly without a shell or package manager.

2. Step 2: Identify common cause of immediate exit

   If the app binary lacks executable permission, the container will exit immediately without running the app.

3. Final Answer:

   The app binary is missing executable permissions -> Option D

4. Quick Check:

   Missing exec permission = container exits immediately [OK]

Hint: Check app permissions; distroless runs binary directly [OK]

Common Mistakes:

• Thinking distroless needs a shell to run commands
• Assuming CMD syntax differs for distroless
• Believing package manager is required to start

5. You want to build a secure production Docker image for a Go app using distroless. Which multi-stage Dockerfile snippet correctly builds and runs the app with minimal image size and attack surface?

hard

A. FROM golang:1.20 AS build
WORKDIR /app
COPY . .
RUN go build -o app
FROM gcr.io/distroless/base
COPY --from=build /app/app /app
CMD ["/app"]

B. FROM golang:1.20
WORKDIR /app
COPY . .
RUN go build -o app
CMD ["./app"]

C. FROM alpine:latest
WORKDIR /app
COPY . .
RUN go build -o app
CMD ["./app"]

D. FROM gcr.io/distroless/base
COPY app /app
RUN go build -o app
CMD ["/app"]

Solution

1. Step 1: Understand multi-stage build for distroless

   Build the Go app in a full image with build tools, then copy the binary into a minimal distroless image.

2. Step 2: Analyze options for correct multi-stage usage

   Only FROM golang:1.20 AS build WORKDIR /app COPY . . RUN go build -o app FROM gcr.io/distroless/base COPY --from=build /app/app /app CMD ["/app"] uses golang image to build, then copies binary into distroless base image for runtime, minimizing size and attack surface.

3. Final Answer:

   FROM golang:1.20 AS build WORKDIR /app COPY . . RUN go build -o app FROM gcr.io/distroless/base COPY --from=build /app/app /app CMD ["/app"] -> Option A

4. Quick Check:

   Build in full image, run in distroless = secure minimal image [OK]

Hint: Build in full image, copy binary to distroless for runtime [OK]

Common Mistakes:

• Trying to build inside distroless image (no build tools)
• Using single-stage build with full image (larger size)
• Using alpine without distroless for production