added explicit plenum version to ubuntu16 Docker files (fixes container build)

Signed-off-by: Sebastian Schmittner <sebastian.schmittner@eecc.de>

Author: Echsecutor

build/Dockerfile.ubuntu16

@@ -15,6 +15,7 @@ RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys CE7709D068
 RUN bash -c 'echo "deb https://repo.sovrin.org/deb xenial stable" >> /etc/apt/sources.list'
 RUN apt-get update -y && apt-get install -y \
     indy-node=1.12.4 \
+    indy-plenum=1.12.4 \
     && rm -rf /var/lib/apt/lists/*
 
 COPY init_and_run.sh ./