Development container patterns in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of multi-stage Docker build

Consider this Dockerfile using multi-stage build pattern:

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

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

What will be the output when running docker images after building this Dockerfile with docker build -t myapp .?

A Only the 'builder' image remains, tagged as 'myapp', containing all build layers

B Two images: one intermediate 'builder' image and one final 'myapp' image with nginx and app files

C No images are created because multi-stage builds do not produce images

D An image named 'myapp' with a single layer containing only the nginx base and the built app files

💡 Hint

Multi-stage builds keep only the final stage as the image unless you tag intermediate stages.

Multi-stage builds produce a single final image named as specified. Intermediate stages are not saved as separate images unless explicitly tagged.

❓ Configuration

intermediate

Correct Dockerfile pattern for caching dependencies

You want to optimize your Dockerfile to cache dependencies and avoid reinstalling them on every build. Which Dockerfile snippet correctly implements this pattern?

A

COPY . ./
RUN npm install

B

COPY package.json ./
RUN npm install
COPY . ./

C

RUN npm install
COPY package.json ./
COPY . ./

D

COPY . ./
RUN npm install
COPY package.json ./

💡 Hint

Think about which files change less often and how Docker caches layers.

Copying package.json first and running npm install before copying the rest allows Docker to cache the dependencies layer if package.json hasn't changed.

❓ Troubleshoot

advanced

Why does my container fail to start with permission denied?

You have this Dockerfile snippet:

FROM python:3.12-slim
WORKDIR /app
COPY . .
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

When running the container, you get a permission denied error on app.py. What is the most likely cause?

A app.py does not have execute permissions inside the container

B The WORKDIR /app does not exist in the container

C pip install failed and dependencies are missing

D The CMD syntax is incorrect and causes failure

💡 Hint

Check file permissions inside the container for scripts.

COPY preserves file permissions from the host. If app.py lacks execute permission, the container cannot run it, causing permission denied.

🔀 Workflow

advanced

Order of steps in a development container workflow

Arrange these steps in the correct order for a typical development container workflow:
1. Run the container interactively
2. Write or edit code on the host
3. Build the Docker image
4. Mount source code into the container

A 4,1,2,3

B 1,4,2,3

C 1,2,4,3

D 1,2,3,4

💡 Hint

Think about when you build, mount, and edit code during development.

First build the image, then mount the source code, then edit code on the host, finally run the container interactively to see changes live.

✅ Best Practice

expert

Choosing the best base image for a development container

You want a development container for a Node.js app that is fast to build, small in size, and supports debugging tools. Which base image is the best choice?

A node:18-slim

B node:18-alpine

C node:18-buster

D node:18

💡 Hint

Consider trade-offs between image size, build speed, and tool availability.

node:18-slim balances small size and compatibility with debugging tools better than alpine (too minimal) or full images (larger).