Distroless images concept in Docker - Mini Project: Build & Apply

Using Distroless Images in Docker

📖 Scenario: You are working on a small web application that you want to deploy using Docker. To make your application image smaller and more secure, you decide to use a distroless image instead of a full Linux base image.Distroless images contain only the application and its runtime dependencies, without a package manager or shell. This reduces the image size and attack surface.

🎯 Goal: Build a Docker image for a simple Go web server using a distroless base image. You will create a Dockerfile that compiles the Go app and then uses a distroless image to run it.

📋 What You'll Learn

Create a Go source file named main.go with a simple HTTP server.

Write a multi-stage Dockerfile that first builds the Go app using the official Go image.

Use the gcr.io/distroless/base image as the final base image to run the compiled binary.

Expose port 8080 in the Dockerfile.

Run the Go binary as the container's entrypoint.

💡 Why This Matters

🌍 Real World

Distroless images are used in production to reduce Docker image size and improve security by removing unnecessary OS components.

💼 Career

Understanding distroless images is important for DevOps engineers and developers who want to optimize container images for cloud deployments.

1

Create the Go web server source file

Create a file named main.go with a simple HTTP server that listens on port 8080 and responds with "Hello, Distroless!" to any request.

Docker

# Your code here

Hint

Use the net/http package to create a simple web server. The handler should write "Hello, Distroless!" to the response.

2

Start the Dockerfile with the Go build stage

Create a Dockerfile that starts with the official Go image golang:1.20 as the build stage named builder. Copy main.go into the container and build the binary named app using go build -o app main.go.

Docker

package main

import (
	"fmt"
	"net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "Hello, Distroless!")
}

func main() {
	http.HandleFunc("/", handler)
	http.ListenAndServe(":8080", nil)
}

# Your Dockerfile build stage here

Hint

Use a multi-stage build. The first stage uses golang:1.20. Set working directory to /app. Copy main.go and build the binary app.

3

Add the distroless base image and copy the binary

In the same Dockerfile, add a final stage that uses the distroless base image gcr.io/distroless/base. Copy the app binary from the builder stage to /app in the final image. Expose port 8080 and set the entrypoint to run /app.

Docker

# Add final distroless stage below

Hint

The final image uses gcr.io/distroless/base. Copy the binary from the builder stage. Expose port 8080 and set the entrypoint to /app.

4

Build and run the Docker image

Build the Docker image with tag distroless-go using docker build -t distroless-go .. Then run the container mapping port 8080 with docker run -p 8080:8080 distroless-go. Finally, print the output of curl http://localhost:8080.

Docker

# Run the commands below and print the curl output
# Your code here

Hint

After building and running the container, use curl http://localhost:8080 to see the response. It should print "Hello, Distroless!".

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