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DevOps / GitHub Actions Interview Questions

How do you write a custom Docker container action for GitHub Actions?

A Docker container action packages its logic and dependencies in a Docker image, giving complete control over the execution environment. It is ideal when your action requires a specific OS, binary tools not available on the runner, or a compiled language without a portable pre-built binary.

action.yml:

name: 'OWASP Dependency Check'
description: 'Run dependency vulnerability scan inside Docker'
inputs:
  project-name:
    description: 'Project name for the report'
    required: true
runs:
  using: 'docker'
  image: 'Dockerfile'        # build from local Dockerfile
  args:
    - ${{ inputs.project-name }}

Dockerfile:

FROM openjdk:21-slim
RUN apt-get update && apt-get install -y curl unzip     && curl -Lo dc.zip https://github.com/jeremylong/DependencyCheck/releases/download/v9.0.0/dependency-check-9.0.0-release.zip     && unzip dc.zip -d /opt && rm dc.zip

COPY entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

entrypoint.sh:

#!/bin/bash
set -e
PROJECT_NAME="$1"
/opt/dependency-check/bin/dependency-check.sh   --project "$PROJECT_NAME"   --scan /github/workspace   --format HTML   --out /github/workspace/dc-report
echo "report-path=dc-report/dependency-check-report.html" >> "$GITHUB_OUTPUT"

Key differences from a JS action:

  • Docker container actions always run on Linux — they cannot execute on Windows or macOS GitHub-hosted runners.
  • The /github/workspace path inside the container is the checked-out repository.
  • You can also reference a pre-built public image (image: 'docker://alpine:3.19') instead of a local Dockerfile to skip the build step.
  • Container build adds latency (~30–60 s) compared to a JS action that starts instantly.
On which operating systems can Docker container actions run in GitHub Actions?
Inside a Docker container action, which path maps to the checked-out repository workspace?

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