Docker copy file to host from within dockerfile

Question

I am writing a Dockerfile to build an application. I know that you can copy files out of the container manually with

docker cp <containerId>:/file/path/within/container /host/path/target 

(see Copying files from Docker container to host), but I would like to just run

docker build 

and have it dump the build artifacts created by my Dockerfile somewhere on my host file system.

Is there a command I can use within the Dockerfile to copy out of the container and into the host? Like the opposite of COPY?

Answer 1

As per Docker docs, API version 1.40 added

Custom build outputs

Which is intended for what you want:

By default, a local container image is created from the build result. The --output (or -o) flag allows you to override this behavior, and a specify a custom exporter. For example, 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.

For example

$ docker build --output type=tar,dest=out.tar . 

with multi-stage Dockerfile

FROM golang AS build-stage RUN go get -u github.com/LK4D4/vndr  FROM scratch AS export-stage COPY --from=build-stage /go/bin/vndr / 

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.

Answer 2

Adding to @Marakai excellent answer, one has to set the env variable DOCKER_BUILDKIT=1. So the command would be:

$ DOCKER_BUILDKIT=1  docker build --output type=tar,dest=out.tar . 

Or one could split into two commands:

$ export DOCKER_BUILDKIT=1 $ docker build --output type=tar,dest=out.tar .  # to gen into a tar file 

Or to save the files without forming a tar file:

$ mkdir out $ docker build  --output out . 

The files will be copied to the directory out.