Fix Dockerfiles to add missing apt-update before apt-install

Change-Id: I639aed00e74c64e770566ce50de10e193e8b8d01
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/docker/MON/Dockerfile b/docker/MON/Dockerfile
index fbb0600..696949a 100644
--- a/docker/MON/Dockerfile
+++ b/docker/MON/Dockerfile
@@ -31,7 +31,8 @@
     python3-setuptools=45.2.* \
     curl=7.68.*
 
-RUN DEBIAN_FRONTEND=noninteractive apt-get --yes install \
+RUN DEBIAN_FRONTEND=noninteractive apt-get --yes update && \
+    DEBIAN_FRONTEND=noninteractive apt-get --yes install \
     openssh-client=1:8.*
 
 ARG PYTHON3_OSM_COMMON_URL