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/tests/Dockerfile b/docker/tests/Dockerfile
index 0c1de38..4e6560f 100644
--- a/docker/tests/Dockerfile
+++ b/docker/tests/Dockerfile
@@ -26,7 +26,8 @@
     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 \
     gettext-base=0.19.* \
     git=1:2.25.* \
     iputils-ping=3:* \