Fix slave-debian Dockerfile setup of OpenJDK 8

Add the automatic installation of the openjdk-8
to Dockerfile without having to request interactive
configuration, which otherwise would result in
failing the build phase.

Change-Id: Id757efbf9de17b8400891ef1f0c7325d6cc9d37a
diff --git a/jenkins-docker/slave-debian/Dockerfile b/jenkins-docker/slave-debian/Dockerfile
index 51ac131..4f85cc9 100644
--- a/jenkins-docker/slave-debian/Dockerfile
+++ b/jenkins-docker/slave-debian/Dockerfile
@@ -74,7 +74,7 @@
 # TODO: to be removed when Gerrit v3.2 will be EOL
 RUN wget -qO - https://adoptopenjdk.jfrog.io/adoptopenjdk/api/gpg/key/public | apt-key add - && \
     add-apt-repository --yes https://adoptopenjdk.jfrog.io/adoptopenjdk/deb/ && \
-    apt-get update && apt-get install adoptopenjdk-8-hotspot && \
+    apt-get update && apt-get install -y adoptopenjdk-8-hotspot && \
     ln -sf /usr/lib/jvm/adoptopenjdk-8-hotspot-amd64 /usr/lib/jvm/java-8-openjdk-amd64 && \
     ln -sf /usr/lib/jvm/adoptopenjdk-8-hotspot-amd64/bin/java /etc/alternatives/java