Dockerfile: bring build up to date with that in azure-pipelines.yml

Update Dockerfile to use Ubuntu 20.04 and the updated package list from
azure-pipelines.yml. Also explicitly include the g++ package into the container
without which configure will error out with a 'configure: error: C++ preprocessor
"/lib/cpp" fails sanity check' error.

Dockerfile

@@ -1,12 +1,12 @@
 # vim:ft=dockerfile
 
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 
-RUN apt-get update && apt-get -y install \
-    g++ \
-    cmake libgmp-dev libmpfr-dev libmpc-dev libboost-all-dev bison \
-    zlib1g-dev \
-    perl texinfo
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y \
+        cmake libgmp-dev libmpfr-dev libmpc-dev \
+        libboost-all-dev bison texinfo \
+        ruby flex curl g++
 
 ADD . /Retro68