make nginx-provisioning work and rename make targets to prefix jenkins-... (#183)

Co-authored-by: Michael Hoennig <michael@hoennig.de>
Reviewed-on: https://dev.hostsharing.net/hostsharing/hs.hsadmin.ng/pulls/183
Reviewed-by: Timotheus Pokorra <timotheus.pokorra@hostsharing.net>

Jenkins/jenkins-agent/Dockerfile

@@ -1,4 +1,10 @@
 FROM eclipse-temurin:21-jdk
+
+# create mount point for jenkins_home
+RUN mkdir -p /var/jenkins_home && \
+    chmod 755 /var/jenkins_home
+
+# install required packages
 RUN apt-get update && \
     apt-get install -y \
         postgresql-client \
@@ -7,3 +13,6 @@ RUN apt-get update && \
         pandoc && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
+
+# continue with the same uid as the user 'jenkins' from the Jenkins Dockerfile
+USER 1000