GitHub Actions with Docker - Mini Project: Build & Apply

GitHub Actions with Docker

📖 Scenario: You work in a small team that wants to automate building a Docker image for your app whenever code is pushed to GitHub. You will create a GitHub Actions workflow file that builds a Docker image using a simple Dockerfile.

🎯 Goal: Build a GitHub Actions workflow file named docker-build.yml that triggers on pushes to the main branch, builds a Docker image from the provided Dockerfile, and tags it with latest.

📋 What You'll Learn

Create a basic Dockerfile with a simple command

Create a GitHub Actions workflow file named docker-build.yml

Configure the workflow to trigger on pushes to main

Add a job that builds the Docker image using the Dockerfile

Tag the Docker image as latest

💡 Why This Matters

🌍 Real World

Automating Docker image builds on code changes helps teams deliver software faster and with fewer errors.

💼 Career

Knowing how to use GitHub Actions with Docker is a key skill for DevOps engineers and developers working with containerized applications.

1

Create a simple Dockerfile

Create a file named Dockerfile with these exact contents:
FROM alpine:latest
CMD ["echo", "Hello from Docker"]

Docker

# Create a Dockerfile with FROM alpine:latest and CMD ["echo", "Hello from Docker"]

Hint

Use FROM alpine:latest to start from a small Linux image.
Use CMD ["echo", "Hello from Docker"] to print a message when the container runs.

2

Create GitHub Actions workflow file

Create a file named .github/workflows/docker-build.yml and add the following header lines:
name: Docker Build
on:
push:
branches: [main]

Docker

FROM alpine:latest
CMD ["echo", "Hello from Docker"]

# Create .github/workflows/docker-build.yml with name and on push to main branch
# Your code here

Hint

Use name: Docker Build to name the workflow.
Use on: push: branches: [main] to trigger on pushes to main branch.

3

Add job to build Docker image

In .github/workflows/docker-build.yml, add a job named build that runs on ubuntu-latest and uses these steps:
1. actions/checkout@v3
2. docker build -t myapp:latest . run in a run step

Docker

name: Docker Build
on:
  push:
    branches: [main]

# Add a job named build that runs on ubuntu-latest
# Add steps to checkout code and build Docker image tagged myapp:latest
# Your code here

Hint

Use jobs: to define jobs.
Use runs-on: ubuntu-latest to specify the runner.
Use actions/checkout@v3 to get the code.
Use docker build -t myapp:latest . to build the image.

4

Print confirmation message

Add a final step in the build job that prints Docker image built successfully using a run step.

Docker

name: Docker Build
on:
  push:
    branches: [main]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Build Docker image
        run: docker build -t myapp:latest .

# Add a step to print 'Docker image built successfully'
# Your code here

Hint

Add a step with run: echo "Docker image built successfully" to show success.

Practice

1. What is the main purpose of using GitHub Actions with Docker in a project?

easy

A. To automate building and testing Docker images on code changes

B. To manually run Docker commands on your local machine

C. To replace Docker with a different container tool

D. To write Dockerfiles without using any commands

Solution

1. Step 1: Understand GitHub Actions role

   GitHub Actions automates tasks like building and testing when code changes happen.

2. Step 2: Connect automation with Docker

   Using GitHub Actions with Docker means automating Docker image builds and tests on events like pushes.

3. Final Answer:

   To automate building and testing Docker images on code changes -> Option A

4. Quick Check:

   Automation with Docker = D [OK]

Hint: GitHub Actions automate tasks triggered by code events [OK]

Common Mistakes:

• Thinking GitHub Actions replace Docker itself
• Confusing manual commands with automation
• Assuming Dockerfiles are written by GitHub Actions

2. Which of the following is the correct syntax to build a Docker image inside a GitHub Actions workflow step?

easy

A. run: docker start -t myimage .

B. run: docker create myimage .

C. run: docker push -t myimage .

D. run: docker build -t myimage .

Solution

1. Step 1: Recall Docker build command

   The command to build an image is 'docker build' with '-t' to tag the image.

2. Step 2: Match with GitHub Actions step syntax

   In GitHub Actions, 'run:' is used to run shell commands, so 'run: docker build -t myimage .' is correct.

3. Final Answer:

   run: docker build -t myimage . -> Option D

4. Quick Check:

   Build image command = C [OK]

Hint: Use 'docker build -t name .' to build images [OK]

Common Mistakes:

• Using 'docker create' instead of 'docker build'
• Confusing 'docker start' with building images
• Using 'docker push' to build images

3. Given this GitHub Actions workflow step:

run: |
  docker build -t test-image .
  docker run --rm test-image echo Hello

What will be the output when this step runs successfully?

medium

A. test-image

B. Hello

C. docker build completed

D. Error: image not found

Solution

1. Step 1: Understand the commands run

   The first command builds an image tagged 'test-image'. The second runs a container from it that prints 'Hello'.

2. Step 2: Identify the output of the run command

   The 'docker run' command runs 'echo Hello' inside the container, so the output is 'Hello'.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   docker run echo output = Hello [OK]

Hint: Run command output is what container prints [OK]

Common Mistakes:

• Thinking build command outputs 'test-image'
• Expecting build success message as output
• Assuming image not found error without build

4. You have this GitHub Actions step:

run: docker build -t myapp .
run: docker run myapp

The workflow fails with 'Error: Dockerfile not found'. What is the likely cause?

medium

A. The Dockerfile is missing in the repository

B. The two 'run' commands are separate steps and do not share state

C. The image name 'myapp' is invalid

D. The 'docker run' command syntax is incorrect

Solution

1. Step 1: Understand docker build requirement

   The 'docker build .' command requires a Dockerfile in the current directory (repo root after checkout).

2. Step 2: Identify the error cause

   If the Dockerfile is missing from the repository, the build step fails with 'Dockerfile not found' error.

3. Final Answer:

   The Dockerfile is missing in the repository -> Option A

4. Quick Check:

   docker build fails = no Dockerfile [OK]

Hint: Ensure Dockerfile exists in repository root [OK]

Common Mistakes:

• Thinking separate steps prevent image sharing
• Assuming image name 'myapp' is invalid
• Believing 'docker run' syntax causes the error

5. You want to create a GitHub Actions workflow that builds a Docker image only when a push happens to the 'main' branch and then pushes the image to Docker Hub. Which of these YAML snippets correctly sets the trigger and steps?

hard

A. on:
push:
tags:
- main
jobs:
build:
runs-on: ubuntu-latest
steps:
- run: docker build -t myuser/myapp .
- run: docker push myuser/myapp

B. on:
pull_request:
branches:
- main
jobs:
build:
runs-on: ubuntu-latest
steps:
- run: docker build -t myapp .
- run: docker push myapp

C. on:
push:
branches:
- main
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v3
- run: docker build -t myuser/myapp:latest .
- run: echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin
- run: docker push myuser/myapp:latest

D. on:
push:
branches:
- develop
jobs:
build:
runs-on: ubuntu-latest
steps:
- run: docker build -t myuser/myapp .
- run: docker push myuser/myapp

Solution

1. Step 1: Check trigger event and branch

   on: push: branches: - main jobs: build: runs-on: ubuntu-latest steps: - uses: actions/checkout@v3 - run: docker build -t myuser/myapp:latest . - run: echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin - run: docker push myuser/myapp:latest triggers on push to 'main' branch, which matches the requirement.

2. Step 2: Verify build and push steps

   on: push: branches: - main jobs: build: runs-on: ubuntu-latest steps: - uses: actions/checkout@v3 - run: docker build -t myuser/myapp:latest . - run: echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin - run: docker push myuser/myapp:latest includes checkout, build with proper tagging, login using secrets, and push commands correctly.

3. Final Answer:

   The snippet that triggers on push to main, checks out code, logs in with secrets, builds tagged image, and pushes -> Option C

4. Quick Check:

   Push on main + login + build + push = A [OK]

Hint: Use 'on: push: branches: - main' and login before push [OK]

Common Mistakes:

• Using pull_request instead of push event
• Triggering on tags instead of branches
• Pushing without Docker login step