How to execute npm commands inside a Docker container on a Kubernetes pod from a Jenkins pipeline?

I am trying to run sh commands from a Jenkins pipeline inside a Docker container, which is located inside a Kubernetes pod.

I am using the Kubernetes plugin and after some research and experimenting, it looks like the commands indeed run in the container, but the workspace is being "replaced" with the one from Jenkins.
In the yaml object that is getting printed in Console Output, one can see a workspace-volume being inserted:

- ...
  - mountPath: "/home/jenkins/agent"
    name: "workspace-volume"
    readOnly: false
- emptyDir:
  medium: ""
name: "workspace-volume"

I need to be able to run npm commands inside the container while the same is working.
How should I execute this?

Btw, I am using Minikube, the image used is already present in Minikube, in Pipeline Steps I can clearly see that the commands are getting executed inside the container, and the Jenkinsfile in question:

pipeline {
  agent any

  stages {
    stage('Test') {
      agent {
        kubernetes {
          yaml '''
apiVersion: v1
kind: Pod
    jenkins: slave
    - name: auth
      image: auth
        - containerPort: 8000
          hostPort: 8000
      tty: true
        - name: MONGODB_TEST
          value: localhost:27017
        - name: PORT
          value: "8000"
        - name: JWT_KEY
          value: "test123"
        - name: JWT_EXPIRES_IN
          value: 7d
      imagePullPolicy: Never

      steps {
        container('auth') {
          sh "ls -la" // workspace is from the pipeline, not the container itself
          sh "npm run test" // fail
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