Get build files to persist on host after docker-compose build is run

Question

I'm trying to run a docker-compose build command with a Dockerfile and a docker-compose.yml file.

Inside the docker-compose.yml file, I'm trying to bind a local folder on the host machine ./dist with a folder on the container app/dist.

version: '3.8'
services:
  dev:
    build:
      context: .
    volumes:
      - ./dist:app/dist # I'm expecting files to be changed or added to the container's app/dist to be reflected to the host's ./dist folder

Inside the Dockerfile, I build some files with an NPM script that I'm wanting to make available on the host machine once the build is finished. I'm also touching a new file inside the /app/dist/test.md just as a simple test to see if the file ends up on the host machine, but it does not.

FROM node:8.17.0-alpine as example

RUN mkdir /app
WORKDIR /app
COPY . /app

RUN npm install
RUN npm run dist
RUN touch /app/dist/test.md

Is there a way to do this? I also tried using the "long syntax" as mentioned in the Docker Compose v3 documentation: https://docs.docker.com/compose/compose-file/compose-file-v3/

question from:https://stackoverflow.com/questions/65879628/get-build-files-to-persist-on-host-after-docker-compose-build-is-run

Answer 1

The easiest way to do this is to install Node and run the npm commands directly on the host.

$BREW_OR_APT_GET_OR_YUM_OR_SOMETHING install node
npm install
npm run dist
# done

There's not an easy way to use a Dockerfile to build host content. The Dockerfile can't write out directly to the host filesystem; if you use a volume mount, the host volume hides the container content before anything else happens.

That means, if you want to use this approach, you need to launch a temporary container to get the content out. You can do it with a one-off container, mounting the host directory somewhere other than /app, making the main container command be cp:

sudo docker build -t myimage .
sudo docker run --rm 
  -v "$PWD/dist:/out" 
  myimage 
  cp -a /app/dist /out

Or, if you specifically wanted to use docker cp:

sudo docker build -t myimage .
sudo docker create --name to-copy myimage
sudo docker cp -r to-copy:/app/dist ./dist
sudo docker rm to-copy

Note that any of these sequences are more complex than just installing a local Node via a package manager, and require administrator permissions (you can use the same technique to overwrite any host file, including the /etc/shadow file with encrypted passwords).