Jenkins Pipeline NPM Integration creates empty package.json - docker

Question

We're running a dedicated build server on Debian 10. Jenkins is installed on there as well as Docker.
The Jenkins user of course owns the jenkins folder and is part of the Docker group. So all rights should be set.
There are already a couple pipelines building Docker images just fine.
Now we're trying to set up a new pipeline to build a web front-end. It's using the "Pipeline NPM Integration" plugin.
The pipeline looks like this
pipeline {
environment {
applicationVersion = '0'
}
agent any
stages {
stage('Clone Git') {
steps {
git url: '<githuburl>',
credentialsId: '<githubcreds>',
branch: 'develop'
}
}
stage('Build module (npm)') {
agent {
docker {
image 'node:15.2-buster-slim'
}
}
steps {
withNPM(npmrcConfig:'af7db72c-3235-4827-8eb4-69819a44e612') {
sh 'npm install'
sh 'npm run build:dev'
}
}
}
}
}
The pipeline always fails at the npm build with the following output
Running on Jenkins in /var/lib/jenkins/workspace/test#2
+ docker inspect -f . node:15.2-buster-slim
.
Jenkins does not seem to be running inside a container
$ docker run -t -d -u 108:114 -w /var/lib/jenkins/workspace/test#2 -v /var/lib/jenkins/workspace/test#2:/var/lib/jenkins/workspace/test#2:rw,z -v /var/lib/jenkins/workspace/test#2#tmp:/var/lib/jenkins/workspace/test#2#tmp:rw,z -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** -e ******** node:15.2-buster-slim cat
$ docker top c227e7fe6de64d5d7046f5565f109e90ecdd9ece1caf4203e6a6f810cc72f40c -eo pid,comm
Using settings config with name npmrc
A workscape local .npmrc already exists and will be overwrriten for the build.
Writing .npmrc file: /var/lib/jenkins/workspace/test#2/.npmrc
+ npm install
up to date in 178ms
found 0 vulnerabilities
+ npm run build:dev
npm ERR! missing script: build:dev
$ docker stop --time=1 c227e7fe6de64d5d7046f5565f109e90ecdd9ece1caf4203e6a6f810cc72f40c
$ docker rm -f c227e7fe6de64d5d7046f5565f109e90ecdd9ece1caf4203e6a6f810cc72f40c
Removed some lines here to keep it short.
After a couple hours I found out that the package.json while being there is actually empty!
cat /var/lib/jenkins/workspace/test#2/package.json
{}
I have no idea why the plugin creates an empty copy of the file.
Is there any way to get more output from the plugin about what it's doing and what could possibly cause this?