Introduction to Docker Containers with a C Application

This example demonstrates how to package a simple C application into a Docker container.

The repository contains:

• helloworld.c — a simple application displaying information about its execution environment.
• Dockerfile.single — a single-stage build.
• Dockerfile.multistage — a multi-stage build.

Learning Objectives

After completing this example, you should be able to:

• Build a Docker image.
• Run a container.
• Understand the purpose of FROM, COPY, RUN, WORKDIR, USER, CMD, and ENTRYPOINT.
• Compare single-stage and multi-stage builds.
• Explain why multi-stage builds produce smaller runtime images.

The Application

The program prints information about its execution environment:

• Container hostname
• Process identifier (PID)
• User identifier (UID)
• An optional environment variable (NAME)

Example output:

Hostname : 8c43b0a0d77b
PID      : 1
UID      : 999
Hello World!

Single-Stage Build

Build the image:

docker build \
    -f Dockerfile.single \
    -t hello:single .

Run the container:

docker run --rm hello:single

Override the environment variable:

docker run --rm \
    -e NAME=Alice \
    hello:single

Example output:

Hostname : 8c43b0a0d77b
PID      : 1
UID      : 999
Hello Alice!

Multi-Stage Build

Build the image:

docker build \
    -f Dockerfile.multistage \
    -t hello:multi .

Run the container:

docker run --rm hello:multi

The application behaves exactly the same, but the runtime image contains only the executable and the required runtime components.

Compare the Images

List the images:

docker images hello

or

docker image ls

Notice that the multi-stage image is significantly smaller because it does not include the compiler and development tools.

You can also inspect the image layers:

docker history hello:single

docker history hello:multi

Experiments

Try the following commands.

Change the greeting

docker run --rm \
    -e NAME=Docker \
    hello:multi

Change the hostname

docker run --rm \
    --hostname=my-container \
    hello:multi

Observe the running user

The Dockerfile creates a non-root user.

Verify the displayed UID and compare it with an image running as the root user.