Docker Containerization Dockerfile and Image Building

Question 1

Which Dockerfile instruction specifies the base image for a new build?

Accepted Answer

FROM

Answer

The FROM instruction is mandatory and must be the first instruction in a Dockerfile; it sets the base image from which you are building.

Question 2

What is the key difference between the ADD and COPY instructions in a Dockerfile?

Accepted Answer

ADD supports remote URLs and auto-extracts tar archives; COPY only copies local files as-is

Answer

ADD has additional functionality: it can fetch files from remote URLs and automatically extract compressed archives like .tar.gz, while COPY is a simpler, more transparent instruction for local files.

Question 3

Which Dockerfile instruction sets the working directory for all subsequent RUN, CMD, ENTRYPOINT, COPY, and ADD instructions?

Accepted Answer

WORKDIR

Answer

WORKDIR sets the working directory inside the container, and if the directory does not exist it will be created automatically.

Question 4

What is the purpose of a .dockerignore file?

Accepted Answer

To specify files and directories to exclude from the build context sent to the Docker daemon

Answer

A .dockerignore file lists patterns of files and directories to exclude from the build context, which reduces build time and prevents sensitive files from being inadvertently added to the image.

Question 5

Which Dockerfile instruction sets a persistent environment variable that is available both during the build and when the container runs?

Accepted Answer

ENV

Answer

ENV sets environment variables that persist in the final image and are accessible to running containers, unlike ARG which is only available at build time.

Docker Practice Test

Docker Practice Test

Docker Containerization Dockerfile and Image Building