Update dist-x86_64-musl to Ubuntu 20.04

Author: nikic

src/ci/docker/host-x86_64/dist-x86_64-musl/Dockerfile

@@ -1,6 +1,6 @@
-FROM ubuntu:16.04
+FROM ubuntu:20.04
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
   g++ \
   make \
   ninja-build \