Docker build (Windows) hangs after RUN 'npm install'

Question

As the title says, I am trying to build a Dockerfile with 'NPM install' as follows:

FROM mcr.microsoft.com/windows/nanoserver:1809 AS build-nodejs
WORKDIR /app
COPY . .

ARG NODE_VERSION=12.10.0

RUN echo "Downloading NodeJS version %NODE_VERSION% ..." && \
    curl "https://nodejs.org/dist/v%NODE_VERSION%/node-v%NODE_VERSION%-win-x64.zip" --output nodejs.zip && \
    echo "Expanding NodeJS ..." && \
    tar -xvf nodejs.zip -C "C:\\" 

RUN SET "PATH=%PATH%C:\node-v%NODE_VERSION%-win-x64" && \
    SET NODE_OPTIONS="--max_old_space_size=4096" && \
    SET CI=true && \
    ECHO "npm install ..." && \
    npm install
 .
 . 
 .

MOVING TO THE FOLLOWING 'FROM' STEP TAKES 15MIN!

FROM mcr.microsoft.com/dotnet/core/sdk:3.0.100 AS build-netcore
WORKDIR /app
.
.
.

What happens between the RUN command and the FROM step?
Why it is taking 15 minutes to move on to the FROM step? I suspect the node_modules creation inside the image to be the cause, probably it gets passed to the FROM section somewhy.
I might be wrong of course and would love to hear more opinions!

Answer 1

So after I have added:

RUN del /S /Q node_modules

After "npm install", leaping between RUN and FROM steps took only 3min and not 15min as it was before.
As I suspected, "node_modules" directory that got created in the RUN's layer delayed the transformation between the layers.
I simply don't understand the process behind what gets passed between layers, if someone could enlight me it will be very helpful.

Answer 2

There are only two chances. FROM will download the image but only first time on the same box.
Your copy. . Will take time if your directly is huge in content.