ARG and ENV instructions in Docker - Mini Project: Build & Apply

Using ARG and ENV Instructions in Dockerfiles

📖 Scenario: You are creating a Docker image for a simple web application. You want to use build-time variables and environment variables inside the Dockerfile to customize the image.

🎯 Goal: Build a Dockerfile that uses ARG to accept a build-time variable and ENV to set an environment variable inside the container.

📋 What You'll Learn

Create a build argument named APP_VERSION with default value 1.0

Set an environment variable named APP_ENV with value production

Use the APP_VERSION argument to label the image

Print the values of APP_VERSION and APP_ENV inside the container

💡 Why This Matters

🌍 Real World

Dockerfiles often need to accept variables at build time and set environment variables for the running container. This project shows how to do both simply.

💼 Career

Understanding ARG and ENV is essential for DevOps roles that involve containerization and building Docker images for deployment.

1

Create a Dockerfile with base image

Create a Dockerfile starting with the base image alpine:latest.

Docker

# Start your Dockerfile with the base image alpine:latest
# Your code here

Need a hint?

The first line in a Dockerfile usually specifies the base image using FROM.

2

Add ARG instruction for build-time variable

Add an ARG instruction named APP_VERSION with default value 1.0.

Docker

FROM alpine:latest
# Add ARG instruction for APP_VERSION with default 1.0
# Your code here

Need a hint?

Use ARG APP_VERSION=1.0 to define a build argument with a default value.

3

Use ARG to label image and add ENV instructions

Add a LABEL instruction using APP_VERSION to label the image, ENV APP_VERSION=${APP_VERSION} to make it available at runtime, and ENV APP_ENV=production.

Docker

FROM alpine:latest
ARG APP_VERSION=1.0
# Add LABEL using APP_VERSION, ENV APP_VERSION=${APP_VERSION}, ENV APP_ENV=production
# Your code here

Need a hint?

Use LABEL version=${APP_VERSION} to label with the ARG, ENV APP_VERSION=${APP_VERSION} to expose ARG at runtime, and ENV APP_ENV=production.

4

Print ARG and ENV values in container

Add a CMD instruction to print the values of APP_VERSION and APP_ENV using echo.

Docker

FROM alpine:latest
ARG APP_VERSION=1.0
LABEL version=${APP_VERSION}
ENV APP_VERSION=${APP_VERSION}
ENV APP_ENV=production
# Add CMD to print APP_VERSION and APP_ENV
# Your code here

Need a hint?

Use CMD echo "App version is $APP_VERSION and environment is $APP_ENV" to print the variables.