[tensorflow] Fix missing apt-get update

tensorflow/Dockerfile

@@ -6,7 +6,8 @@ LABEL authors="Moritz Maxeiner <moritz.maxeiner@fu-berlin.de>"
 
 ARG DEBIAN_FRONTEND=noninteractive
 
-RUN apt-get install -y \
+RUN apt-get update && \
+    apt-get install -y \
         openjdk-11-jdk \
         swig \
         libcurl3-dev \