FluentsDevExam Prep

/Dockerfile & Build System

Concept Detail

Dockerfile & Build System

Difficulty: medium

Overview

A Dockerfile is a text script of instructions that Docker executes sequentially to build an image.

Core Instructions:

Instruction	Creates Layer?	Purpose
FROM	Yes	Base image — must be first instruction
RUN	Yes	Execute shell commands during build
COPY	Yes	Copy files from build context
ADD	Yes	Like COPY + URL download + tar extraction
CMD	No	Default command (overridable at `docker run`)
ENTRYPOINT	No	Fixed executable; CMD becomes its args
ENV	No*	Persistent environment variable
ARG	No	Build-time variable; not in final image
EXPOSE	No	Document listening port (does not publish)
VOLUME	Yes	Declare anonymous mount point
WORKDIR	No*	Set working directory
USER	No*	Switch to non-root user
LABEL	No*	Add metadata key-value pairs
HEALTHCHECK	No	Define container health check

CMD vs ENTRYPOINT:

CMD sets defaults — easily overridden: docker run image custom-cmd.
ENTRYPOINT fixes the executable — docker run args become parameters to it.
Together: ENTRYPOINT ["nginx"] + CMD ["-g","daemon off;"] is the idiomatic pattern.

Exec form vs Shell form:

Exec: ["executable","arg"] — no shell, signals reach the process, PID 1 is your app. Preferred.
Shell: command arg — wrapped in /bin/sh -c, PID 1 is the shell, SIGTERM not forwarded.

Build Cache:
Docker caches each layer. Cache is reused if the instruction text and all preceding layers are unchanged. For COPY/ADD, the file contents are also compared. Order instructions from least-changing to most-changing for maximum cache efficiency.

Multi-stage Builds:

FROM node:20 AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM nginx:alpine
COPY --from=builder /app/dist /usr/share/nginx/html

The final image contains only nginx + dist — no node_modules, build tools, or source code.

Practice Linked Questions

easy

Q1. What is the purpose of the FROM instruction in a Dockerfile?

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easy

Q2. What does the RUN instruction do in a Dockerfile?

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medium

Q3. What is the difference between the COPY and ADD instructions in a Dockerfile?

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medium

Q4. What is the difference between CMD and ENTRYPOINT in a Dockerfile?

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hard

Q5. A Dockerfile has these instructions in order: FROM, RUN apt-get install, COPY package.json, RUN npm install, COPY . . — You change only the application source code. Which layers will Docker rebuild from cache?

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medium

Q6. What is the purpose of the ENV instruction in a Dockerfile?

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medium

Q7. What is the key difference between ARG and ENV in a Dockerfile?

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medium

Q8. What is the VOLUME instruction used for in a Dockerfile?

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medium

Q9. Why should containers avoid running as the root user, and which Dockerfile instruction sets the user?

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hard

Q10. What is a multi-stage build in Docker and what problem does it solve?

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hard

Q11. What is the difference between the exec form and shell form of CMD and ENTRYPOINT?

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medium

Q12. What is the purpose of a .dockerignore file?

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hard

Q13. What does the HEALTHCHECK instruction do in a Dockerfile?

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hard

Q14. You have `ENTRYPOINT ["python", "app.py"]` and `CMD ["--port", "8080"]` in a Dockerfile. What happens when you run `docker run myimage --port 9090`?

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medium

Q15. Which Dockerfile instructions create a new image layer? (Select all that apply — more than one answer may be correct.)

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medium

Q16. Which of the following are valid forms of the CMD instruction? (Select all that apply — more than one answer may be correct.)

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hard

Q17. Which practices reduce Docker image size? (Select all that apply — more than one answer may be correct.)

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easy

Q18. What is the LABEL instruction used for in a Dockerfile?

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hard

Q19. What is the ONBUILD instruction used for?

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