How to copy files from a docker image - dockerfile cmd

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During build time, I want to copy a file from the image (from folder /opt/myApp/myFile.xml), to my host folder /opt/temp In the Dockerfile, I'm using the --mount as follows, trying to mount to my local test folder:

RUN --mount=target=/opt/temp,type=bind,source=test cp /opt/myApp/myFile.xml /opt/temp

I'm building the image successfully, but the local test folder is empty any ideas?

dockerfile docker mount
1 Answers

Copying files from the image to the host at build-time is not supported.
This can easily be achieved during run-time using volumes.

However, if you really want to work-around this by all means, you can have a look in the custom build outputs documentation, that introduced support for this kind of activity.

Here is a simple example inspired from the official documentation:

Dockerfile

FROM alpine AS stage-a
RUN mkdir -p /opt/temp/
RUN touch /opt/temp/file-created-at-build-time
RUN echo "Content added at build-time" > /opt/temp/file-created-at-build-time

FROM scratch as custom-exporter
COPY --from=stage-a /opt/temp/file-created-at-build-time .

For this to work, you need to launch the build command using these arguments:

DOCKER_BUILDKIT=1 docker build --output out .

This will create on your host, aside the Dockerfile, a directory out with the file you need:

.
├── Dockerfile
└── out
    └── file-created-at-build-time

cat out/file-created-at-build-time 
Content added at build-time
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