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What is Dockerfile?

A Dockerfile is a text-based script that defines the instructions for building a Docker container image, introduced alongside Docker by Solomon Hykes and dotCloud in 2013. Each instruction in a Dockerfile — FROM, RUN, COPY, ENV, EXPOSE, CMD — creates a layer in the resulting image, and the order and content of these instructions directly affect image size, build speed, and runtime security. Docker revolutionized software deployment by enabling applications to run in isolated, reproducible containers across any environment, from developer laptops to production Kubernetes clusters.

Dockerfiles are central to CI/CD pipelines, where they define how applications are packaged for deployment on platforms like AWS ECS, Google Cloud Run, Azure Container Instances, and self-hosted infrastructure. Multi-stage builds allow developers to separate build-time dependencies from the final runtime image, dramatically reducing image sizes. The ecosystem includes Docker Compose for multi-container orchestration, BuildKit for advanced build features, and registries like Docker Hub, GitHub Container Registry, and Amazon ECR for image distribution.

Security scanning tools like Trivy, Snyk, and Docker Scout analyze Dockerfile contents and resulting images for vulnerabilities. Best practices emphasize using official base images, minimizing layers, running as non-root users, and leveraging .dockerignore files. Dockerfiles remain the primary mechanism for defining container images, even as alternative tools like Buildpacks and Nix gain traction.