ARG and ENV instructions in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of ARG and ENV in Dockerfile build

Given the Dockerfile below, what will be the output of the command docker build . when it prints the environment variable?

Docker

FROM alpine
ARG APP_VERSION=1.0
ENV APP_VERSION=$APP_VERSION
RUN echo "App version is $APP_VERSION"

A App version is null

B App version is

C App version is $APP_VERSION

D App version is 1.0

💡 Hint

Remember ARG is available only during build, ENV sets environment variables for runtime and build.

The ARG APP_VERSION is set to 1.0 during build. The ENV instruction sets APP_VERSION to the ARG value, so the RUN command prints the value 1.0.

🧠 Conceptual

intermediate

Difference between ARG and ENV in Dockerfile

Which statement correctly describes the difference between ARG and ENV instructions in a Dockerfile?

A ARG is only available during build time; ENV is available during build and runtime.

B ENV is only available during build time; ARG is available during build and runtime.

C Both ARG and ENV are only available during runtime.

D ARG and ENV are identical and interchangeable.

💡 Hint

Think about when you can access these variables: during image build or when running the container.

ARG variables exist only during the image build process. ENV variables persist in the image and are available when the container runs.

❓ Troubleshoot

advanced

Why does ENV not get the ARG value at runtime?

Consider this Dockerfile snippet:

ARG PORT=8080
ENV PORT=80
CMD echo $PORT

What will be the output when running the container, and why?

A Empty output, because PORT is not set at runtime.

B 8080, because ARG PORT is used at runtime.

C 80, because ENV PORT overrides ARG PORT and is used at runtime.

D Syntax error during build.

💡 Hint

ENV sets the environment variable inside the image, ARG does not persist after build.

ENV PORT=80 sets the environment variable PORT inside the image. ARG PORT=8080 is only available during build and does not affect runtime environment variables.

🔀 Workflow

advanced

Passing build-time ARG and runtime ENV variables

You want to build a Docker image with a build-time variable VERSION defaulting to 1.0, and also allow overriding a runtime environment variable APP_MODE when running the container. Which Dockerfile and run command combination achieves this?

A

Dockerfile:
ENV VERSION=1.0
ARG APP_MODE=production
RUN echo $APP_MODE

Run: docker run -e VERSION=development image

B

Dockerfile:
ARG VERSION=1.0
ENV APP_MODE=production
RUN echo $VERSION

Run: docker run -e APP_MODE=development image

C

Dockerfile:
ARG VERSION=1.0
ENV APP_MODE=production
RUN echo $APP_MODE

Run: docker run image

D

Dockerfile:
ENV VERSION=1.0
ENV APP_MODE=production
RUN echo $VERSION

Run: docker run -e APP_MODE=development image

💡 Hint

Remember ARG is for build-time, ENV sets default runtime variables, and -e overrides runtime variables.

ARG VERSION=1.0 sets a build-time variable. ENV APP_MODE=production sets a default runtime variable. The run command overrides APP_MODE with development.

✅ Best Practice

expert

Secure handling of sensitive data with ARG and ENV

Which approach is best to avoid leaking sensitive data like passwords in Docker images using ARG and ENV?

A Use ARG for sensitive data and do not set it as ENV to avoid storing it in the image layers.

B Use ENV for sensitive data so it is available at runtime and build time.

C Hardcode sensitive data in the Dockerfile for consistency.

D Use ENV and ARG together to store sensitive data in both build and runtime.

💡 Hint

Think about which variables persist in the final image layers and which do not.

ARG variables are not persisted in the final image layers, so sensitive data passed as ARG is not stored in the image. ENV variables are stored in image layers and can be seen by anyone with the image.