Multi-stage for different environments in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of multi-stage Docker build with environment-specific stages

Given this Dockerfile with two stages named dev and prod, what will be the output of docker build --target prod . when running the container?

Docker

FROM alpine AS dev
RUN echo "Development build" > /build.txt

FROM alpine AS prod
RUN echo "Production build" > /build.txt
CMD cat /build.txt

FROM prod

A Production build

B Error: No such file /build.txt

C Development build

D Empty output

💡 Hint

The final stage is prod, so the file content comes from that stage.

The build uses the prod stage as the final stage. The file /build.txt is created in the prod stage with content "Production build". The container runs cat /build.txt, so it outputs "Production build".

❓ Configuration

intermediate

Correct multi-stage Dockerfile for dev and prod with shared base

Which Dockerfile correctly uses a shared base stage and separate dev and prod stages that add environment-specific files?

A

FROM node:18 AS base
WORKDIR /app
COPY package.json .
RUN npm install

FROM base
COPY dev.config.js prod.config.js .
CMD ["node", "app.js"]

B

FROM node:18
WORKDIR /app
COPY package.json .
RUN npm install
COPY dev.config.js .
CMD ["node", "app.js"]

FROM node:18
COPY prod.config.js .
CMD ["node", "app.js"]

C

FROM node:18 AS base
WORKDIR /app
COPY package.json .
RUN npm install

FROM base AS dev
COPY dev.config.js .
CMD ["node", "app.js"]

FROM base AS prod
COPY prod.config.js .
CMD ["node", "app.js"]

D

FROM node:18 AS base
WORKDIR /app
COPY package.json .
RUN npm install

FROM dev
COPY dev.config.js .
CMD ["node", "app.js"]

FROM prod
COPY prod.config.js .
CMD ["node", "app.js"]

💡 Hint

Look for a shared base stage and separate dev and prod stages that copy only their config files.

Option C correctly defines a base stage with common setup, then separate dev and prod stages that copy their specific config files and set the command. Other options either lack base stage reuse or have invalid stage references.

❓ Troubleshoot

advanced

Why does the prod stage fail to find a file from the base stage?

Given this Dockerfile snippet, why does the prod stage fail with "file not found" error? FROM alpine AS base RUN echo "Hello" > /greeting.txt FROM alpine AS prod COPY /greeting.txt /greeting.txt CMD cat /greeting.txt

A COPY command in prod stage tries to copy from local build context, not from base stage

B File /greeting.txt does not exist in base stage

C CMD syntax is incorrect and causes failure

D Alpine image does not support COPY command

💡 Hint

COPY without --from copies from build context, not from other stages.

COPY without --from copies files from the build context (your local folder), not from other build stages. To copy from base stage, you must use COPY --from=base /greeting.txt /greeting.txt.

🔀 Workflow

advanced

Best workflow to build and push dev and prod images using multi-stage Dockerfile

You have a multi-stage Dockerfile with stages dev and prod. Which sequence of commands correctly builds and pushes both images tagged for dev and prod?

A

docker build --target dev -t myapp:dev .
docker build --target prod -t myapp:prod .
docker push myapp:dev
# forgot to push prod

B

docker build -t myapp:dev .
docker build -t myapp:prod .
docker push myapp:dev
docker push myapp:prod

C

docker build --target prod -t myapp:prod .
docker push myapp:prod
docker build --target dev -t myapp:dev .
docker push myapp:dev

D

docker build --target dev -t myapp:dev .
docker push myapp:dev
docker build --target prod -t myapp:prod .
docker push myapp:prod

💡 Hint

Build and push each target image separately with correct tags.

Option D builds each target stage separately with tags and pushes both images. Option D builds without specifying target so both builds produce the same final stage image. Option D forgets to push prod image. Option D builds prod first then dev, which is valid but order is less common; however, both A and D build and push both images correctly, but only A matches the recommended order for dev then prod.

🧠 Conceptual

expert

Why use multi-stage builds for different environments in Docker?

What is the main advantage of using multi-stage Docker builds with separate stages for dev and prod environments?

A It forces all environment files to be included in every image, increasing consistency

B It allows creating smaller, optimized images for each environment by sharing common layers

C It makes the build process slower but easier to debug

D It requires separate Dockerfiles for each environment, improving clarity

💡 Hint

Think about image size and reuse of common parts.

Multi-stage builds let you share common setup in a base stage and create separate smaller images for dev and prod by including only what each needs. This reduces image size and improves build efficiency.