Docker Docker & Containers: From Zero to Production Docker Images Dockerfile Module 2 — Docker Images

Dockerfile Instructions — FROM, RUN, COPY, ADD, and More

Reviewed & accurate

AI Summary

R Ram Pothuraju September 23, 2026 Reading…

What You'll Learn

Beginner

Every Dockerfile instruction and when to use it
The difference between CMD and ENTRYPOINT
COPY vs ADD — which to use when

Complete Instruction Reference

Instruction	Purpose	Layer?
`FROM`	Base image	Yes
`RUN`	Execute during build	Yes
`COPY`	Copy files from host	Yes
`ADD`	Like COPY + extract tarballs	Yes
`WORKDIR`	Set working directory	No
`ENV`	Set environment variable	No
`ARG`	Build-time variable	No
`EXPOSE`	Document port	No
`CMD`	Default command	No
`ENTRYPOINT`	Fixed command	No

COPY vs ADD

Rule of thumb

Use COPY for everything. Only use ADD when you need to extract a tarball.

Feature	COPY	ADD
Copy local files	Yes	Yes
Extract tarballs	No	Yes
Recommended?	Yes	Avoid

CMD vs ENTRYPOINT

Most confused topic

Both define what runs when the container starts. CMD is overridable; ENTRYPOINT is fixed.

Aspect	CMD	ENTRYPOINT
Purpose	Default command + args	Fixed command
`docker run image arg`	Replaces CMD	Appends to ENTRYPOINT
Best for	Generic images	Specific apps

The best pattern: ENTRYPOINT + CMD

Dockerfiledockerfile

ENTRYPOINT ["node"]
CMD ["server.js"]

# docker run myapp          -> node server.js
# docker run myapp test.js  -> node test.js

USER — Run as Non-Root

Security best practice

By default, containers run as root. Create a non-root user and switch to it.

Dockerfiledockerfile

RUN addgroup -S appgroup && adduser -S appuser -G appgroup
USER appuser

Practical Exercise

Review the Dockerfile instructions table

Write a Dockerfile with FROM, COPY, RUN, CMD

Test CMD vs ENTRYPOINT behavior

Add a non-root USER

Key Takeaways

FROM sets the base. RUN executes during build. COPY adds files.
Use COPY over ADD.
ENV persists at runtime; ARG is build-time only.
CMD is overridable; ENTRYPOINT is fixed. Use both for flexibility.
Always run as non-root with USER.

Previously: Lesson 11 built your first image.
Today: You learned: Every Dockerfile instruction explained with examples.
Next: Lesson 13 covers best practices.

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What You'll Learn

Complete Instruction Reference

COPY vs ADD

CMD vs ENTRYPOINT

The best pattern: ENTRYPOINT + CMD

USER — Run as Non-Root

Practical Exercise

Key Takeaways

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