200-901 Application Deployment and Security Practice Question
A developer creates a Dockerfile for a Python web application. Which instruction should be used to copy the application source code into the container image?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
COPY
The COPY instruction copies files or directories from the build context into the container filesystem. RUN executes commands, EXPOSE documents ports, and CMD sets default command.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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CMD
Why it's wrong here
CMD sets the default command executed when the container starts; it never transfers files into the image. It is tempting because it configures runtime behaviour, which is essential in a Dockerfile, but the correct instruction for copying source code from the build context is COPY.
- ✓
COPY
Why this is correct
COPY transfers files from the build context into the image filesystem, so the Python source lands in the image. ADD also copies but additionally handles remote URLs and archive extraction, which the stem does not require.
- ✗
RUN
Why it's wrong here
RUN executes a command during the image build and does not transfer files from the build context into the image. It is tempting because RUN is used constantly in Dockerfiles for installing packages, but copying source code requires COPY (or ADD), which reads from the build context and writes into the image filesystem.
- ✗
EXPOSE
Why it's wrong here
EXPOSE documents which port the container listens on at runtime; it transfers no files into the image. It is tempting because it is a legitimate Dockerfile instruction for network configuration, but copying application source code from the build context requires COPY.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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