Add new GMP dependency to dockerfile

Docker.ubuntu20

diff --git a/Docker.ubuntu20 b/Docker.ubuntu20
index 8171fa2..aee2b6a 100644
--- a/Docker.ubuntu20
+++ b/Docker.ubuntu20
@@ -8,7 +8,7 @@
 
 RUN apt-get update
 RUN DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get -y install tzdata
-RUN apt-get install -y cmake bison flex libboost-all-dev python2 perl
+RUN apt-get install -y cmake bison flex libboost-all-dev libgmp-dev python2 perl
 RUN apt-get install -y git
 RUN apt-get install -y g++ gcc 
 RUN git clone https://github.com/stp/stp