200-901 Application Deployment and Security Practice Question
A developer wants to create a Docker image that runs a Python application. Which instruction should be placed at the end of the Dockerfile to specify the command that runs when the container starts?
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
✓
CMD
CMD specifies the default command to run when the container starts, and it can be overridden by providing a command at runtime. ENTRYPOINT is similar but not easily overridden. RUN executes during build. COPY copies files during build.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
RUN
Why it's wrong here
RUN executes commands during image build, baking results into a layer, and never runs at container start. It is tempting because it appears to execute the Python application, but that execution happens only while building; the runtime start command requires CMD or ENTRYPOINT instead.
- ✗
ENTRYPOINT
Why it's wrong here
ENTRYPOINT sets the executable that always runs, but the stem asks for the instruction specifying the command at container start; CMD supplies default arguments and is overridden at runtime. ENTRYPOINT is tempting because it also defines startup behaviour, and would be correct when the container must always run one fixed executable.
- ✗
COPY
Why it's wrong here
COPY transfers files from the build context into the image filesystem during build; it defines no runtime process. It is tempting because the Python application files must be present in the image, but copying them is a prerequisite step, not the instruction that launches the application when the container starts.
- ✓
CMD
Why this is correct
CMD sets the default executable for the container, supplying the command that runs at startup. Placed last, it defines the Python application's launch process, and unlike ENTRYPOINT it can be overridden at runtime with docker run arguments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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