Merge branch 'update-mono-6' into 'master'

Add build-essential to mono 6 base image

See merge request common/dockerfiles!52

base/mono:6

@@ -5,5 +5,5 @@ LABEL author="Tamas Gal <tamas.gal@fau.de>"
 RUN apt-get update \
   && apt-get install -y \
       binutils curl mono-devel ca-certificates-mono fsharp mono-vbnc nuget referenceassemblies-pcl \
-      make \
+      build-essential make \
   && rm -rf /var/lib/apt/lists/* /tmp/*