containerized Jenkins (#179)

Co-authored-by: Michael Hoennig <michael@hoennig.de>
Reviewed-on: https://dev.hostsharing.net/hostsharing/hs.hsadmin.ng/pulls/179
Reviewed-by: Timotheus Pokorra <timotheus.pokorra@hostsharing.net>

Jenkins/agent/Dockerfile

@@ -0,0 +1,9 @@
+FROM eclipse-temurin:21-jdk
+RUN apt-get update && \
+    apt-get install -y \
+        postgresql-client \
+        bind9-utils \
+        docker.io \
+        pandoc && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*