Cannot install npm package in a mounted directory

Question

My idea is to use docker container for building js and css assets separately from an app. I have code at the host machine and I'm trying to mount the directory with my code as volume to docker container. But in the Dockerfile when I do RUN npm install sematic-ui --save it doesn't change anything in the directory I'm trying to mount.

Is the mounted volume available at all at the building process or should I install npm packages only when I run image?

Also what is the best practice in my case?

EDIT:

I simplified the case to show an example. Here are my files.

docker-compose.yml

version: '3'
services:
 web:
  build: .
  volumes:
    - ./vol:/root/test

Dockerfile

FROM debian:9
WORKDIR /root/test
RUN touch index.html
CMD /bin/sh

Then I do docker-compose build . I expect to see the index.html in the vol directory on the host. Why this doesn't happen? As I noticed some docker images could change the host's filesystem during build. But I couldn't.

Answer 1

When you mount a volume, it replaces everything in the directory to which you are mounting it in the container. Whatever you have in /root/test will be replaced with your host ./vol directory.

---

Also I'm not sure this would work anyway because when these lines happen:

WORKDIR /root/test
RUN touch index.html

the /root/test directory doesn't exist. I'm not sure where Docker is putting the index.html. Typically I will RUN mkdir -p /root/test to ensure the container path is explicitly created.