Add docker instalation.

Dockerfile

 FROM jenkins/inbound-agent:4.3-4-jdk11
 
-# install maven like https://github.com/carlossg/docker-maven/tree/master/openjdk-11
+USER root
+
+# Install maven like in https://github.com/carlossg/docker-maven/tree/master/openjdk-11 .
 ARG MAVEN_VERSION=3.6.3
-ARG USER_HOME_DIR="/root"
+ARG USER_HOME_DIR="/home/jenkins"
 ARG SHA=c35a1803a6e70a126e80b2b3ae33eed961f83ed74d18fcd16909b2d44d7dada3203f1ffe726c17ef8dcca2dcaa9fca676987befeadc9b9f759967a8cb77181c0
 ARG BASE_URL=https://apache.osuosl.org/maven/maven-3/${MAVEN_VERSION}/binaries
-
 RUN mkdir -p /usr/share/maven /usr/share/maven/ref \
   && curl -fsSL -o /tmp/apache-maven.tar.gz ${BASE_URL}/apache-maven-${MAVEN_VERSION}-bin.tar.gz \
   && echo "${SHA}  /tmp/apache-maven.tar.gz" | sha512sum -c - \
   && tar -xzf /tmp/apache-maven.tar.gz -C /usr/share/maven --strip-components=1 \
   && rm -f /tmp/apache-maven.tar.gz \
   && ln -s /usr/share/maven/bin/mvn /usr/bin/mvn
-
 ENV MAVEN_HOME /usr/share/maven
 ENV MAVEN_CONFIG "$USER_HOME_DIR/.m2"
-
 COPY mvn-entrypoint.sh /usr/local/bin/mvn-entrypoint.sh
 COPY settings-docker.xml /usr/share/maven/ref/
-
-# configure maven
 COPY settings-maven.xml ${MAVEN_CONFIG}/settings.xml
 
+# Install Docker like in https://medium.com/@manav503/how-to-build-docker-images-inside-a-jenkins-container-d59944102f30 .
+RUN apt-get update -qq \
+    && apt-get install -qqy apt-transport-https ca-certificates curl gnupg2 software-properties-common 
+RUN curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -
+RUN add-apt-repository \
+   "deb [arch=amd64] https://download.docker.com/linux/debian \
+   $(lsb_release -cs) \
+   stable"
+RUN apt-get update  -qq \
+    && apt-get install docker-ce=18.06.3~ce~3-0~debian -y
+RUN usermod -aG docker jenkins
+
+# Configure Maven.
+RUN chown -R jenkins:jenkins ${MAVEN_CONFIG}
+USER jenkins
+
+# Configure Docker.
+#   Nothing to do.
+
 ENTRYPOINT ["/usr/local/bin/mvn-entrypoint.sh"]
 
 CMD ["mvn"]