Image layer

As explained in .NET in Action, Second Edition

A container image layer is a separate immutable filesystem change produced by an image-build step and stored as part of the image.

An image layer is a separate, immutable filesystem change produced by an image-build step and stored as part of a container image. Docker typically creates one layer per Dockerfile instruction, allowing unchanged steps to be reused from cache. section 13.3.1

Use layers deliberately when structuring a Dockerfile:

• Split large build operations into smaller steps so more work can be cached. section 13.3.1
• Copy a project file and run dotnet restore before copying source files; unchanged project files preserve the restored-package layer. section 13.3.1
• Use a multi-stage Dockerfile to publish with the SDK image and copy only the published output into the runtime image. section 13.3.1

Figure 13.4 Docker Desktop with the asciiartsvc image open to the Layers section

Inspect the image's Layers section to see the separate build results retained in the container image. section 13.3.1

How .NET in Action, Second Edition covers it

How deep it goes

Explains layer creation, cache invalidation, Dockerfile ordering, and multi-stage composition without detailing storage internals.

Examples

Uses a .NET 6 Dockerfile, a separate restore step, and Docker Desktop's Layers section as concrete examples.

What it assumes

Assumes Dockerfile and container-image familiarity while demonstrating layer behavior through .NET publishing workflows.

What you can do after

Apply it by ordering Dockerfile steps to preserve cached dependencies and produce smaller multi-stage images.

How it connects

Connects image layers with Dockerfiles, Docker build cache, dotnet restore, and multi-stage builds.

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Also explained in

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  Learn Docker in a Month of Lunches, Second EditionAn image layer is an immutable filesystem change and associated data that forms part of a container image and can be shared by other images and containers.
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  Kubernetes in Action, Second EditionA container image layer is an independently stored filesystem change that composes a container image and can be shared across images.
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  Podman in Action: Secure, rootless containers for Kubernetes, microservices, and moreAn image layer is an immutable filesystem change set that contributes to a container image and can be stacked with other layers.
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  Build Python Web Apps with StreamlitA Docker image layer is an immutable filesystem change that contributes to the assembled contents of a Docker image.

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Related concepts

• container imagepart of · A container image is a packaged filesystem and configuration artifact from which a container can be created and run.
• Dockerfileproduced by · A Dockerfile is a text file containing declarative instructions for building a container image and specifying its contents and runtime metadata.
• Dockerfile instructionproduced by · A Dockerfile instruction is a declarative directive that specifies a build operation or image metadata during container image construction.
• run instructionproduced by · A Dockerfile RUN instruction executes a command during container image construction, producing filesystem changes that become part of the resulting image.
• virtual filesystempart of · A virtual filesystem is a software abstraction that presents files and directories through filesystem-like organization and operations without requiring direct access to physical storage.
• container registryused by · A container registry is a service or repository for storing, managing, and distributing container images and related artifacts.

This explainer was generated by Manning's knowledge graph from the text of .NET in Action, Second Edition. The section links point to the parts of the book it draws on.