Understanding Docker: Containers, Images, Registries, and Core Linux Technologies

Docker is a foundational cloud‑native technology and a core component of cloud computing; this article gives a detailed explanation of Docker’s underlying principles.

Docker Containers are lightweight, isolated runtime instances of images, similar to class objects in programming, providing process isolation and resource control.

Docker Images are immutable, layered packages that contain all code, runtime, libraries, environment variables, and configuration needed to run an application; they can be viewed as static snapshots or install packages.

Docker
Image
├── Layer1: FROM ubuntu
├── Layer2: RUN apt install nginx
├── Layer3: COPY index.html /var/www
└── ... (multiple layers form a multi‑layer image)

Docker Containers run on top of an image by adding a writable layer; the container’s processes have separate namespaces and resource limits, and the writable layer disappears when the container is removed.

Docker Registry is a storage service for images, similar to a code repository; users can push images to or pull images from public (e.g., Docker Hub) or private registries.

The core Linux technologies that make Docker work are:

Namespaces : PID, NET, IPC, UTS, MNT, USER – provide isolation of processes, network, inter‑process communication, hostnames, file system mounts, and user IDs.

Cgroups : limit and monitor CPU, memory, I/O, and network bandwidth, enforce resource isolation, and prevent a container from exhausting host resources.

UnionFS (e.g., OverlayFS) : implements a layered, read‑only image system with a writable top layer, enabling fast image construction and startup.

Example of limiting a container to 512 MB of memory:

docker run -m 512m nginx

In summary, Docker leverages Linux namespaces for isolation, cgroups for resource control, and UnionFS for efficient image management, forming the basis of modern container‑based deployment workflows.