Revert docker image to 18.04 since ld fails on later versions

Author: koalaman

Dockerfile

@@ -1,5 +1,5 @@
 # Build-only image
-FROM ubuntu:19.10 AS build
+FROM ubuntu:18.04 AS build
 USER root
 WORKDIR /opt/shellCheck
 

manpage

@@ -1,4 +1,4 @@
 #!/bin/sh
-echo >&2 "Generating man page using pandoc" 
+echo >&2 "Generating man page using pandoc"
 pandoc -s -f markdown-smart -t man shellcheck.1.md -o shellcheck.1 || exit
 echo >&2 "Done. You can read it with:   man ./shellcheck.1"