Glossary — Docker explained: images, containers and volumes
Key terms explained
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- Image
- A read-only, layered filesystem snapshot built from a Dockerfile. Each RUN, COPY, and ADD instruction adds a new layer. Images are stored in registries and identified by a name, tag, and content digest.
- Container
- A running process isolated using Linux namespaces and cgroups, sharing the host kernel but with its own filesystem, network, and process tree. Containers are ephemeral by default — stopping one discards its writable layer.
- Volume
- A Docker-managed persistent storage unit that exists outside the container's writable layer. Volumes survive container removal, can be shared between containers, and are the recommended way to persist database files and application state.
- Layer
- A read-only filesystem diff produced by a single Dockerfile instruction. Layers are cached and shared between images that use the same base. Ordering instructions from least to most frequently changed maximises cache reuse and speeds builds.
- Registry
- A server that stores and distributes Docker images. Docker Hub is the default public registry. Private registries (GitHub Container Registry, AWS ECR, Azure ACR) are used for proprietary images. Push with 'docker push'; pull with 'docker pull'.
- Bind Mount
- Mounting a specific host filesystem path directly into a container. Unlike volumes, bind mounts depend on the host directory structure. Useful in development for source-code hot reload, but avoided in production where path portability matters.
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