How to copy files from dockerfile to host?

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This is now possible since Docker 19.03.0 in July 2019 introduced “custom build outputs”. See the official docs about custom build outputs.

To enable custom build outputs from the build image into the host during the build process, you need to activate the BuildKit which is a newer recommended back-compatible way for the engine to do the build phase. See the official docs for enabling BuildKit.

This can be done in 2 ways:

  1. Set the environment variable DOCKER_BUILDKIT=1, or
  2. Set it in the docker engine by default by adding "features": { "buildkit": true } to the root of the config json.

From the official docs about custom build outputs:

custom exporters allow you to export the build artifacts as files on the local filesystem instead of a Docker image, which can be useful for generating local binaries, code generation etc.

…

The local exporter writes the resulting build files to a directory on the client side. The tar exporter is similar but writes the files as a single tarball (.tar).

If no type is specified, the value defaults to the output directory of the local exporter.

…

The –output option exports all files from the target stage. A common pattern for exporting only specific files is to do multi-stage builds and to copy the desired files to a new scratch stage with COPY –from.

e.g. an example Dockerfile

FROM alpine:latest AS stage1
WORKDIR /app
RUN echo "hello world" > output.txt

FROM scratch AS export-stage
COPY --from=stage1 /app/output.txt .

Running

DOCKER_BUILDKIT=1 docker build --file Dockerfile --output out .

The tail of the output is:

 => [export-stage 1/1] COPY --from=stage1 /app/output.txt .
0.0s
 => exporting to client
0.1s
 => => copying files 45B
0.1s

This produces a local file out/output.txt that was created by the RUN command.

$ cat out/output.txt
hello world

All files are output from the target stage

The --output option will export all files from the target stage. So using a non-scratch stage with COPY --from will cause extraneous files to be copied to the output. The recommendation is to use a scratch stage with COPY --from.

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