Upgrade the Emscripten Ubuntu image to 20.04

Upgrade the underlying operating system for a newer CMake version which is necessary to compile Xerces-C++.

emscripten/Dockerfile

-FROM ubuntu:18.04
+FROM ubuntu:20.04
 
 RUN apt-get update && \
     apt-get dist-upgrade -y && \
+    DEBIAN_FRONTEND=noninteractive \
     apt-get install -y \
         build-essential \
         cmake \