What Is Docker? Containers for Build and Deploy Explained
Docker is a tool for packaging applications and their dependencies into portable container images that run the same way everywhere, from a laptop to production.
Docker made containers mainstream. It lets you describe an application's environment in a file, build that into an image, and run it as an isolated container anywhere Docker is installed. In CI/CD, it is both how builds get reproducible environments and how apps get packaged for deployment.
What Docker is
Docker is a platform for building, shipping, and running containers. A container is an isolated process that bundles an application with its libraries and runtime, sharing the host kernel but otherwise separated from other processes. The Docker CLI and daemon build images and run them as containers.
Images and containers
An image is an immutable, layered snapshot of a filesystem and metadata, built from instructions in a Dockerfile. A container is a running instance of an image. Layers are cached and shared, so rebuilding an image only redoes the steps that changed, which is key to fast builds.
A Dockerfile example
A Dockerfile describes how to build the image.
FROM node:20-slim
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
CMD ["npm", "start"]Role in CI/CD
Docker is central to CI/CD: pipelines build images, run tests inside containers for clean reproducible environments, and push images to a registry for deployment. Image builds are I/O and CPU heavy; effective layer caching and a build cache are the biggest levers. These builds run faster and cheaper on managed runners with warm caches plus auto-retry on transient registry pulls.
Alternatives
Podman is a daemonless, rootless alternative with a Docker-compatible CLI. BuildKit and Buildah focus on building images, while containerd is the lower-level runtime. Docker remains the most familiar end-to-end tool for building and running containers in development and CI.
Key takeaways
- Docker packages apps into portable, layered container images.
- Layer caching makes rebuilds fast by redoing only changed steps.
- In CI it builds images, isolates tests, and produces deployable artifacts.