Development container patterns in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to set the base image for a development container.

Docker

FROM [1]

Drag options to blanks, or click blank then click option'

Apython:3.12-slim

Bnode:18-alpine

Cubuntu:latest

Dnginx:stable

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a production-only image like nginx for development.

The FROM instruction sets the base image. Here, node:18-alpine is a lightweight Node.js image suitable for development.

2fill in blank

medium

Complete the Dockerfile command to copy local source code into the container.

Docker

COPY [1] /app

Drag options to blanks, or click blank then click option'

A./

B./src

C/src

Dsrc/

Attempts:

3 left

💡 Hint

Common Mistakes

Using absolute paths like /src which don't exist in the build context.

The COPY ./ /app command copies all files from the current directory into the container's /app folder.

3fill in blank

hard

Fix the error in the Dockerfile command to install dependencies using npm.

Docker

RUN npm [1]

Drag options to blanks, or click blank then click option'

Ainstall

Bupdate

Cupgrade

Dbuild

Attempts:

3 left

💡 Hint

Common Mistakes

Using npm update which updates packages but does not install new ones.

The correct command to install dependencies is npm install. Other options do not install packages.

4fill in blank

hard

Fill both blanks to expose port 3000 and set the default command to start the app.

Docker

EXPOSE [1]
CMD ["npm", "[2]"]

Drag options to blanks, or click blank then click option'

A3000

Bstart

Cbuild

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Exposing the wrong port or using npm build which only compiles code.

Port 3000 is commonly used for development servers. The command npm start runs the start script defined in package.json.

5fill in blank

hard

Fill all three blanks to create a volume mount for live code editing and set environment variable for development mode.

Docker

VOLUME ["[1]"]
ENV NODE_ENV=[2]
CMD ["npm", "[3]"]

Drag options to blanks, or click blank then click option'

A/app

Bdevelopment

Cstart

D/src

Attempts:

3 left

💡 Hint

Common Mistakes

Mounting the wrong directory or forgetting to set NODE_ENV for development.

Mounting /app allows live code changes inside the container. Setting NODE_ENV=development enables dev mode. Running npm start launches the app.

Practice

1. What is the main purpose of using a development container in Docker?

easy

A. To create a consistent coding environment isolated from your main computer

B. To speed up your internet connection

C. To permanently delete files from your computer

D. To run games inside Docker

Solution

1. Step 1: Understand what a development container does

   Development containers provide a consistent environment for coding by isolating tools and dependencies.

2. Step 2: Match the purpose to the options

   Only To create a consistent coding environment isolated from your main computer describes creating a consistent, isolated coding environment.

3. Final Answer:

   To create a consistent coding environment isolated from your main computer -> Option A

4. Quick Check:

   Development container purpose = consistent isolated environment [OK]

Hint: Think: 'Is this about coding environment isolation?' [OK]

Common Mistakes:

• Confusing containers with internet speed tools
• Thinking containers delete files permanently
• Assuming containers run unrelated apps like games

2. Which Dockerfile instruction is used to specify the base image for a development container?

easy

A. RUN

B. CMD

C. FROM

D. COPY

Solution

1. Step 1: Identify Dockerfile instructions

   Dockerfile uses specific instructions like FROM, RUN, COPY, CMD for different purposes.

2. Step 2: Match instruction to base image

   FROM sets the base image; RUN executes commands; COPY copies files; CMD sets default command.

3. Final Answer:

   FROM -> Option C

4. Quick Check:

   Base image instruction = FROM [OK]

Hint: Base image always starts with FROM in Dockerfile [OK]

Common Mistakes:

• Using RUN to set base image
• Confusing COPY with base image
• Thinking CMD sets base image

3. Given this Dockerfile snippet for a development container:

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

What happens when you run this container?

medium

A. The container fails because WORKDIR is missing

B. The container runs but does not install dependencies

C. The container runs but does not start app.py

D. The container runs Python 3.12, installs dependencies, and starts app.py

Solution

1. Step 1: Analyze Dockerfile commands

   FROM sets Python 3.12 base, WORKDIR sets /app, COPY copies files, RUN installs dependencies, CMD runs app.py.

2. Step 2: Predict container behavior

   All steps are correct and complete, so container installs dependencies and runs app.py.

3. Final Answer:

   The container runs Python 3.12, installs dependencies, and starts app.py -> Option D

4. Quick Check:

   Dockerfile commands = install and run app [OK]

Hint: Check each Dockerfile line for its role [OK]

Common Mistakes:

• Ignoring RUN command effect
• Assuming WORKDIR is missing
• Thinking CMD does not start app

4. You wrote this Dockerfile for a development container:

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

But when you build and run, the app crashes with missing modules error. What is the likely problem?

medium

A. You did not copy package-lock.json before npm install

B. The COPY . . after RUN npm install overwrites node_modules with host files if node_modules not in .dockerignore

C. You copied package.json before npm install but did not copy other files before npm install

D. You copied package.json after npm install

Solution

1. Step 1: Review Dockerfile copy and install order

   package.json copied first for npm install (good for caching), then full COPY . . after.

2. Step 2: Identify the overwrite issue

   RUN npm install builds linux-compatible node_modules, but COPY . . copies host files including node_modules/ (wrong platform), overwriting it and causing missing modules or binary errors. Fix: node_modules in .dockerignore.

3. Final Answer:

   The COPY . . after RUN npm install overwrites node_modules with host files if node_modules not in .dockerignore -> Option B

4. Quick Check:

   Node Dockerfile pattern = COPY pkg, install, COPY src, .dockerignore node_modules [OK]

Hint: Copy package.json -> npm install -> COPY . . but .dockerignore node_modules/ [OK]

Common Mistakes:

• Copying package.json after npm install
• Confusing package-lock.json necessity
• Assuming source files must be copied before npm install

5. You want to create a development container that automatically reloads your Node.js app when you change source files on your host machine. Which pattern should you use?

hard

A. Use a volume mount to sync source files and run nodemon inside the container

B. Copy source files once during build and restart container manually on changes

C. Build a new image every time source files change

D. Run the app outside Docker and only use Docker for dependencies

Solution

1. Step 1: Understand live reload needs

   Live reload requires syncing source files between host and container and a watcher like nodemon inside container.

2. Step 2: Match pattern to options

   Volume mount syncs files live; nodemon watches and reloads app automatically.

3. Final Answer:

   Use a volume mount to sync source files and run nodemon inside the container -> Option A

4. Quick Check:

   Live reload = volume mount + nodemon [OK]

Hint: Volume mount + nodemon = auto reload dev container [OK]

Common Mistakes:

• Copying files only once without sync
• Rebuilding image on every change
• Running app outside Docker defeats container purpose