Add deb-multimedia repository in separate step

Dockerfile

@@ -3,7 +3,9 @@ FROM debian:testing
 RUN echo 'deb http://ftp-stud.hs-esslingen.de/pub/Mirrors/debian-multimedia/ testing main' > /etc/apt/sources.list.d/deb-multimedia.list \
     && apt-get update -oAcquire::AllowInsecureRepositories=true \
     && apt-get -y --allow-unauthenticated install deb-multimedia-keyring \
-    && apt-get update \
+    && rm -rf /var/lib/apt/lists
+
+RUN apt-get update \
     && apt-get -y install \
         autoconf \
         beignet-opencl-icd \