Docker build takes long time for nodejs application - node.js

Question

I am experiencing long build times for nodejs applications when building image with docker build command.
There is one big hang that takes couple of minutes
08:03:15 Step 1/11 : FROM node:14.1.0-alpine AS build
08:03:15 ---> 0854fcfc1637
08:03:15 Step 2/11 : COPY server/package*.json /nodejs/server/
08:03:15 ---> Using cache
08:03:15 ---> 4996283ff991
08:03:15 Step 3/11 : WORKDIR /nodejs/server
08:03:15 ---> Using cache
08:03:15 ---> 93e5b63fa81d
08:03:15 Step 4/11 : RUN npm ci
08:03:15 ---> Using cache
08:03:15 ---> 2c825e02ea01
08:03:15 Step 5/11 : COPY server ./
08:03:15 ---> Using cache
08:03:15 ---> 69c024cde79f
08:03:15 Step 6/11 : WORKDIR /nodejs
08:03:15 ---> Using cache
08:03:15 ---> 49d7f8bd9514
08:03:15 Step 7/11 : COPY package*.json ./
08:03:16 ---> e82bee625c3e
08:03:16 Step 8/11 : RUN npm ci
08:03:16 ---> Running in ecfd57702906
...
08:03:49 added 1483 packages in 26.419s
08:09:40 Removing intermediate container ecfd57702906
...
08:09:40 ---> 7c6b67d85b0b
08:09:40 Step 9/11 : COPY *.json ./
08:09:43 ---> 0165efd1c97d
08:09:43 Step 10/11 : COPY src ./src/
08:09:51 ---> 42e54cee6b91
08:09:51 Step 11/11 : RUN npm run build:prod
08:09:51 ---> Running in af6f9b013d27
This does not happen when building Java images.
My Dockerfile
FROM node:14.1.0-alpine AS build
COPY server/package*.json /nodejs/server/
WORKDIR /nodejs/server
RUN npm ci
COPY server ./
WORKDIR /nodejs
COPY package*.json ./
RUN npm ci
COPY *.json ./
COPY src ./src/
RUN npm run build:prod
...
I tried using buildkit but it has the same behaviour
08:37:20 #17 exporting to image
08:37:20 #17 exporting layers
08:50:12 #17 exporting layers 766.8s done
I also added node_modules to .dockerignore file but with no change.
Docker version 19.03.6, build 369ce74a3c
What could be the problem?
What is happening between "added 1483 packages..." and "Removing intermediate container"?

Answer 1

I have found the cause of the problem and wanted to share if someone encounters similar issue.
The long "export layers" time was caused by node_modules folder that was present in one of the layers. Removing it solved the problem but it's really a workaround.
I merged npm ci and npm run build... steps together and removed node_modules in the same step.
RUN npm ci && npm run build:prod && rm -rf node_modules
So the final build stage in Dockerfile looks like this
FROM node:14.1.0-alpine AS build
COPY server/package*.json /nodejs/server/
WORKDIR /nodejs/server
RUN npm ci
COPY server ./
WORKDIR /nodejs
COPY package*.json ./
COPY *.json ./
COPY src ./src/
RUN npm ci && npm run build:prod && rm -rf node_modules
...
As I said it's just a workaround and I think the root cause is that Docker is having problems with exporting layers with a lot of small files such as the ones in node_modules.