I'm using Jenkins with Kubernetes plugin and I want to combine JNLP image with my slave image

I want my default JNLP container to run with my image.
Therefore, I need to overrite the JNLP with my image, but to keep the JNLP data to be able to connect to my master.

1. The jnlp image I should have as base is inbound-agent no?
2. How can I combine it with my image if I already have "FROM UBUNTU" can I combine multiple base images and copy the artifacts? How can I do that and what should be my dockerfile?

My own image -

FROM ubuntu:18.04
ARG JFROG_CI_INT_USERNAME
ARG JFROG_CI_INT_PASSWORD
ARG JFROG_CI_INT_NPM_TOKEN
ARG GITHUB_ORANGE_USER
ARG GITHUB_ORANGE_PASSWORD
ARG PULUMI_USER
ARG PULUMI_TOKEN
ENV GITHUB_ORANGE_USER=$GITHUB_ORANGE_USER
ENV GITHUB_ORANGE_PASSWORD=$GITHUB_ORANGE_PASSWORD
ENV DEBIAN_FRONTEND=noninteractive
#=============
# Set WORKDIR
#=============
WORKDIR /home/jenkins
COPY requirements.txt /
COPY authorization.sh /
# Update software repository
RUN apt-get update
RUN apt-get -qqy install software-properties-common
RUN add-apt-repository ppa:git-core/ppa
RUN add-apt-repository ppa:openjdk-r/ppa
RUN add-apt-repository ppa:deadsnakes/ppa
RUN apt-get -qqy install git
ENV PYTHON_VER="3.9"
RUN apt-get -qqy update && \
apt-get -qqy --no-install-recommends install \
gpg-agent \
software-properties-common \
openjdk-11-jdk \
ca-certificates \
build-essential \
tzdata \
zip \
unzip \
ENTRYPOINT ["/usr/local/bin/start.sh"]

You can do a multi-stage build but you'll end with a tightly coupled image, and you'll have to rebuild it every time you want to change the jenkins agent version.

There's a better (IMHO) option, using two containers. You can run an agent in kubernetes using two images: inbound-agent and your image. This is from a working pipeline that I have:

pipeline {
    agent {
        kubernetes {
            yaml """
apiVersion: v1
kind: Pod
spec:
    containers:
    - name: jnlp
      image: jenkins/inbound-agent:4.10-3-alpine-jdk8
      volumeMounts:
      - name: home-volume
        mountPath: /home/jenkins
      env:
      - name: HOME
        value: /home/jenkins
    - name: maven
      image: my-registry:5000/maven-3.6.3-jdk-11:latest
      command:
      - sleep
      args: 
      - 1d
      volumeMounts:
      - name: home-volume
        mountPath: /home/jenkins
      env:
      - name: JAVA_TOOL_OPTIONS
        value: -Dfile.encoding=UTF8
    volumes:
    - name: home-volume
      emptyDir: {}
"""
        }
    }
    stages {
      stage('Build') {
        steps {
          script {
            container('maven') {
              sh('mvn clean deploy')

This way you have both images decoupled, but they run together in the same pod to make the pipeline work.