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A container image packages a filesystem description and execution configuration used to create software containers. It contains such things as application files, libraries and a default command. Image layers and content identifiers permit sharing unchanged content; a running container adds execution state and usually a separate writable layer.^[ref]

For example, two containers can start from the same image while receiving different input files and environment settings. Their results need not match merely because their image matches. A container image digest identifies particular image content, while a movable tag can later refer to a different image. Most containers share a host kernel, so an image is not necessarily a complete guest operating-system installation. Reproducing a calculation also requires its inputs, configuration and compatible host facilities.

A container image digest identifies particular container image content through a cryptographic hash function, commonly written as an algorithm name followed by a hexadecimal value. Unlike a movable tag, a digest fixes the referenced content. Image manifests identify the configuration and layers; multi-platform indexes can refer to several platform-specific manifests.^[ref]

For example, a tag such as analysis:stable can point to different content next month. Recording an image reference with a digest makes that change detectable and allows the same referenced content to be requested again. It does not establish who produced the image, whether its software is correct, or that required input data and host facilities are identical. Those claims need separate evidence.

A software container runs one or more operating-system processes with a controlled view of files and other operating system resources. A container image supplies a packaged filesystem and execution configuration; starting it creates running operating-system processes with additional runtime state. A software container typically shares the host operating-system kernel, whereas a virtual machine can run its own operating-system kernel.^[ref]

For example, two analyses can use different versions of the same software dependency without placing both versions in one shared installation. Reusing a container image helps reproduce those dependencies. Results also depend on the input data, configuration, mounted files and compatible host facilities. Packaging the software therefore records part of a reproducible calculation; the scientific assumptions and numerical methods still need their own justification.