200-901 Application Deployment and Security Practice Question
A developer creates a Dockerfile with the following content: FROM python:3.9-slim, COPY app.py /app/, RUN pip install flask, EXPOSE 5000, CMD ["python", "/app/app.py"]. When building the image with 'docker build -t myapp .', what is the purpose of the EXPOSE instruction?
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
✓
It informs Docker that the container listens on port 5000, but the port must be published at runtime.
EXPOSE documents that the container listens on port 5000 at runtime. It does not publish the port; that requires -p flag when running the container.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It automatically publishes port 5000 to a random host port.
Why it's wrong here
EXPOSE does not publish ports; -p flag is needed.
- ✓
It informs Docker that the container listens on port 5000, but the port must be published at runtime.
Why this is correct
EXPOSE documents the port, but publishing requires -p.
- ✗
It installs the Flask framework on port 5000.
Why it's wrong here
Flask is installed by RUN pip, not by EXPOSE.
- ✗
It maps host port 5000 to container port 5000.
Why it's wrong here
Mapping requires -p flag.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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