How to Extract Files from Docker Container

Running the Docker Image in a Container

We'll run the image in a Docker container using docker run command. The container will be assigned a name (${container_name}) so that it can be referred later. The path to the build script will be passed as the command (${cmd}) to docker run. This will override the CMD command in the Dockerfile and the script will be executed.

docker run \
    --name ${container_name} \
    ${image} \
    ${cmd}

Once the script execution is completed, the container will be stopped. All modifications to the filesystem will be retained in the container.

Copy Data from Stopped Docker Container

Any file from the Docker container's filesystem can be copied to the host system using docker cp command. We need to pass the container name (${container_name}), path to the file (${path_to_copy}), and destination location in the host system (${output_dir}) where to copy the content.

docker cp ${container_name}:${path_to_copy} ${output_dir}

Updated Docker Script

A new function extract() is added to the docker.sh script that will run a Docker image in a container and extract desired file(s) out of it.

#!/usr/bin/bash

# Run the docker image in a container and extract output
extract()
{
  repo="${1:-${DEFAULT_REPO}}"
  tag="${2:-${DEFAULT_TAG}}"
  image=${repo}:${tag}
  container_name="cont-$(date +%s)"

  # The variables below should be updated to suit a project's need
  cmd=/src/starter/build.sh
  path_to_copy=/src/starter/build/bin
  output_dir=./out

  # Run the container
  # Pass the command to execute, this will override CMD in the Dockerfile
  docker run \
    --name ${container_name} \
    ${image} \
    ${cmd}

  rm -rf ${output_dir}
  mkdir -p ${output_dir}
  
  # Copy files from the container filesystem to host
  docker cp ${container_name}:${path_to_copy} ${output_dir}

  # Data copied, delete the container
  docker container rm --force --volumes ${container_name}
}

Once the image is ready, it can be run in a container and necessary files can be copied to host system using the following command:

./docker.sh extract <repo> <tag>

The extract() function will delete the container and all filesystems associated with it if all docker command execution is successful.