Docker Containerization Dockerfile and Image Building 3

Question 1

What is the difference between the ARG and ENV instructions in a Dockerfile?

Accepted Answer

ARG is only available during the build process; ENV persists in the final image and running containers

Answer

ARG defines build-time variables passed via --build-arg that are not available in the running container, while ENV variables persist into the final image and are accessible at runtime.

Question 2

What is the key behavioral difference between ENTRYPOINT and CMD in a Dockerfile?

Accepted Answer

ENTRYPOINT defines a fixed executable that is not overridden by 'docker run' arguments; CMD provides defaults that are replaced by those arguments

Answer

ENTRYPOINT sets the main executable and is not overridden when you pass arguments to 'docker run' (those arguments are appended instead), while CMD defaults are replaced entirely by any arguments passed to 'docker run'.

Question 3

Which Dockerfile instruction is used to switch the user context for subsequent RUN, CMD, and ENTRYPOINT instructions?

Accepted Answer

USER

Answer

The USER instruction sets the user (and optionally group) that will be used when running the image, which is a security best practice to avoid running containers as root.

Question 4

What is the Docker build context?

Accepted Answer

The set of files and directories sent to the Docker daemon when you run 'docker build'

Answer

The build context is the contents of the directory (or URL) passed to 'docker build'; Docker sends these files to the daemon so they can be referenced in COPY and ADD instructions.

Question 5

What does the HEALTHCHECK instruction in a Dockerfile do?

Accepted Answer

It defines a command Docker runs periodically to test whether the container is functioning correctly

Answer

HEALTHCHECK tells Docker how to periodically probe a running container to determine its health status, which is then reflected in 'docker ps' output and used by orchestrators like Swarm.

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Docker Containerization Dockerfile and Image Building 3