Скачать или смотреть Understanding Multi-Stage Dockerfiles: Do You Need to Copy Files from Stage "A" to Stage "B"?

Discover whether it's necessary to COPY files from one stage to another in a multi-stage Dockerfile. Learn the ins and outs of file reuse with clear explanations and examples!
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Understanding Multi-Stage Dockerfiles: Do You Need to Copy Files from Stage "A" to Stage "B"?

When working with Docker, especially with multi-stage builds, you might find yourself asking a particular question: Do I have to COPY the files from stage "a" to stage "b" if stage "b" is defined as "FROM a AS b"? This question stems from trying to optimize your Dockerfile for efficiency and structure. Let’s break down the answer in detail.

The Premise: Multi-Stage Builds

Multi-stage builds in Docker are an excellent way to streamline your Dockerfile by allowing you to have multiple stages in the build process. The purpose of this approach is to separate the build environment from the run-time environment. Typically, you would use a heavier image (like one containing build tools) in your first stage and then copy only the necessary files to a lighter run-time image.

Example Dockerfile

To illustrate, let’s look at a standard multi-stage Dockerfile example:

[[See Video to Reveal this Text or Code Snippet]]

In this Dockerfile, the initial stage named "build" prepares your application, while the second stage, named "start," is intended to serve or run your application.

Clarifying the Question

You might often wonder: In the start stage, do I need to perform COPY --from=build <...files> again? The concern is that since you defined the start stage as FROM build, it may reuse all the files from that stage.

The Answer: No Need for Redundant COPY

The good news is: you do not need to copy files again. When you define a new stage as FROM build, it does not create a new identical file system – instead, it continues from the build stage. Therefore:

The FROM build statement does not require a COPY --from=build command for the same files.

You can simplify your Dockerfile by omitting the additional COPY commands since the files are already available.

Here’s the simplified Dockerfile:

[[See Video to Reveal this Text or Code Snippet]]

Why Use Multi-Stage Builds?

The primary reason for utilizing multi-stage builds is to create smaller, more efficient final images. Here’s how it typically works:

Stage 1 (build): You compile and build your application using a full-featured image that includes all the required tools for the build process.

Stage 2 (start): You can use a much smaller base image for running your application, thus avoiding bloat from build tools, leading to a lightweight final image.

Key Takeaways

No Redundant Copies: You do not have to COPY files again if the next stage is defined as FROM build.

Efficiency: Multi-stage builds help keep your final Docker images lean and manageable by separating build and run environments.

Simplification: By understanding how stages interact and share file systems, you can write cleaner and more concise Dockerfiles.

By optimizing your Dockerfile with this understanding, you’ll not only enhance your workflow but also ensure that your container images are efficient and well-structured. Happy Dockering!