Clean-up of apt information in docker layers

Change-Id: I25949f4e61d327f61dfee8b7a9c3115738c445ac
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/docker/PLA/Dockerfile b/docker/PLA/Dockerfile
index 712a095..8d4350b 100644
--- a/docker/PLA/Dockerfile
+++ b/docker/PLA/Dockerfile
@@ -46,7 +46,8 @@
 LABEL authors="Lars-Göran Magnusson"
 
 RUN DEBIAN_FRONTEND=noninteractive apt-get --yes update && \
-    DEBIAN_FRONTEND=noninteractive apt-get --yes install python3-minimal=3.6.*
+    DEBIAN_FRONTEND=noninteractive apt-get --yes install python3-minimal=3.6.* \
+    && rm -rf /var/lib/apt/lists/*
 
 COPY --from=INSTALL /usr/lib/python3/dist-packages /usr/lib/python3/dist-packages
 COPY --from=INSTALL /usr/local/lib/python3.6/dist-packages  /usr/local/lib/python3.6/dist-packages