Distroless images concept in Docker - Commands & Configuration

Introduction

Distroless images are minimal container images that contain only your application and its runtime dependencies, without any extra operating system tools or shells. This makes your containers smaller, faster to download, and more secure by reducing attack surface.

When you want to reduce the size of your container images to save bandwidth and storage.

When you want to improve container security by removing unnecessary tools and shells.

When you want to deploy applications with only the essential runtime libraries.

When you want faster startup times for your containers.

When you want to avoid accidental changes or debugging inside the container by removing shells.

Config File - Dockerfile

Dockerfile

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

FROM gcr.io/distroless/base: Uses the minimal distroless base image without a shell or package manager.

COPY my-app /my-app: Copies your compiled application binary into the image.

CMD ["/my-app"]: Sets the command to run your application when the container starts.

Commands

Builds the Docker image named 'my-app-distroless' using the Dockerfile in the current directory.

Terminal

docker build -t my-app-distroless .

Expected Output

Sending build context to Docker daemon  2.56MB

Step 1/3 : FROM gcr.io/distroless/base

 ---> 123abc456def

Step 2/3 : COPY my-app /my-app

 ---> Using cache

 ---> 789def012abc

Step 3/3 : CMD ["/my-app"]

 ---> Using cache

 ---> 456abc789def

Successfully built 456abc789def

Successfully tagged my-app-distroless:latest

Runs the 'my-app-distroless' container and removes it after it stops. This starts your application inside the minimal distroless image.

Terminal

docker run --rm my-app-distroless

Expected Output

Hello from my-app!

→

--rm - Automatically remove the container when it exits

Shows the size and details of the 'my-app-distroless' image to verify it is smaller than typical images.

Terminal

docker images my-app-distroless

Expected Output

REPOSITORY          TAG       IMAGE ID       CREATED         SIZE

my-app-distroless   latest    456abc789def   2 minutes ago   20MB

Key Concept

If you remember nothing else from this pattern, remember: distroless images include only what your app needs to run, making containers smaller and safer.

Common Mistakes

Trying to run shell commands inside a distroless container.

Distroless images do not include shells or package managers, so shell commands will fail.

Build and debug your app outside the container, then run it inside the distroless image without expecting shell access.

Using distroless images for apps that require interactive debugging or shell access.

You cannot open a shell or install tools inside distroless containers, which makes debugging inside the container impossible.

Use distroless images for production deployments and use full base images during development and debugging.

Summary

Build your app into a minimal distroless Docker image using a simple Dockerfile.

Run the container to start your app without extra OS tools or shells.

Check the image size to confirm it is smaller and more secure than typical images.

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