# Knowledge Check: Dockerfile and Image Build

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**1. What does `WORKDIR` actually set?**

*Answer:* The working directory for every subsequent instruction in the build, and the
container's default working directory when it runs.

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**2. What is the "build context"?**

*Answer:* The directory (usually specified as `.` at the end of `docker build -t tag .`) that
Docker can see and copy files FROM during the build — `COPY` instructions read from it.

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**3. Why does instruction order in a Dockerfile affect build speed?**

*Answer:* Docker caches each instruction's result as a layer and reuses cached layers for
unchanged instructions — everything before the first ACTUALLY-changed instruction stays cached,
so putting rarely-changing instructions (like dependency installs) before frequently-changing
ones (like application code) minimizes unnecessary rebuilding.

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**4. What's `.dockerignore` for, and what other file does it resemble?**

*Answer:* It excludes files from the build context (keeping large/irrelevant files out of the
image and speeding up builds) — the same idea as ROOT 03's `.gitignore`.

## Instructor Notes

**Facilitation:** Question 3 directly reinforces the caching demo — a good check that the
concept, not just the observation, landed.

**Approximate timing:** 5 minutes.