Builder pattern before multi-stage in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of multi-stage build simulation using builder pattern

Given the Dockerfile below simulating a builder pattern before multi-stage builds, what will be the output when running docker build .?

Docker

FROM node:18 AS builder
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . ./
RUN npm run build

FROM node:18
WORKDIR /app
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/index.js"]

A Build succeeds but final image contains all source files and node_modules

B Build fails due to COPY --from=builder syntax not supported before multi-stage

C Build succeeds and final image contains only /app/dist with built files

D Build fails because npm install is not run in final stage

💡 Hint

Consider how multi-stage builds isolate build artifacts from final image.

The Dockerfile uses multi-stage build syntax correctly. The first stage named 'builder' installs dependencies and builds the app. The final stage copies only the built files from the builder stage, resulting in a smaller final image.

🧠 Conceptual

intermediate

Why use builder pattern before multi-stage builds?

Before Docker supported multi-stage builds, why was the builder pattern used in Dockerfiles?

A To separate build dependencies from runtime dependencies and reduce final image size

B To enable parallel builds of multiple images simultaneously

C To allow running multiple containers from the same image

D To automatically update base images during build

💡 Hint

Think about how build tools and runtime environments differ.

The builder pattern was used to keep build tools and dependencies separate from the final runtime image, reducing image size and attack surface.

❓ Troubleshoot

advanced

Troubleshooting COPY --from error in Dockerfile

You have this Dockerfile simulating a builder pattern before multi-stage builds:

FROM node:18
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . ./
RUN npm run build

COPY --from=builder /app/dist ./dist
CMD ["node", "dist/index.js"]

What error will Docker produce when building this Dockerfile?

A Error: unknown flag: --from

B Error: missing 'AS builder' stage declaration

C No error, build completes successfully

D Error: COPY failed: stat /app/dist: no such file or directory

💡 Hint

Check if the build stage named 'builder' exists.

The COPY --from=builder requires a build stage named 'builder'. Without 'AS builder' in the first FROM line, Docker cannot find the source stage.

✅ Best Practice

advanced

Best practice for reducing final image size with builder pattern

Which approach best reduces the final Docker image size when using a builder pattern before multi-stage builds?

A Use a separate build container to compile artifacts, then copy only artifacts into a minimal runtime image

B Use a single Dockerfile stage with all build and runtime steps combined

C Install all dependencies in the final image to avoid missing packages

D Include source code and node_modules in the final image for debugging

💡 Hint

Think about what files are necessary to run the app versus build it.

Separating build and runtime environments keeps the final image small and secure by including only necessary runtime files.

🔀 Workflow

expert

Order of steps in builder pattern Dockerfile before multi-stage

Arrange the following Dockerfile steps in the correct order to implement a builder pattern before multi-stage builds:

A 1,3,2,4

B 1,2,3,4

C 2,3,1,4

D 2,1,3,4

💡 Hint

Consider what must be present before building and what is needed in the runtime.

First install dependencies, then copy source code, then build, and finally copy artifacts to runtime container.