Docker ARG vs ENV – Usage Scenarios

🔹 What is ARG?

• ARG is used to pass build-time variables
• Available only during docker build
• Not available inside running container (unless passed to ENV)

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🔹 What is ENV?

• ENV is used to define runtime environment variables
• Available inside the container after it starts

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Real-World Scenarios

1. Dynamic Base Image Version (ARG)

Use ARG when you want to change base image version without modifying Dockerfile

ARG BASE_VERSION=22.04
FROM ubuntu:${BASE_VERSION}

Use Case

Build different versions:

docker build -t app:v1 --build-arg BASE_VERSION=20.04 .
docker build -t app:v2 --build-arg BASE_VERSION=22.04 .

2. Install Packages Dynamically (ARG)

ARG APP=nginx
RUN apt-get update && apt-get install -y ${APP}

Use Case

Install different software using same Dockerfile

3. Application Configuration (ENV)

ENV APP_ENV=production
ENV DB_HOST=localhost

Use Case

Configure app behavior (dev/test/prod

4. Database Connection (ENV)

ENV DB_USER=admin
ENV DB_PASS=password

Use Case

Provide runtime DB configuration

5. Combine ARG + ENV (Best Practice)

ARG APP_ENV=dev
ENV APP_ENV=${APP_ENV}

Use Case

Pass value at build time → use at runtime

docker build -t app --build-arg APP_ENV=production .

6. Debug vs Production Mode

ARG MODE=prod
ENV MODE=${MODE}

Use Case

Enable debug logs or optimizations