Add ncurses to the image so clear works (PowerShell#367)

Author: JamesWTruher

release/preview/centos8/docker/Dockerfile

@@ -32,6 +32,8 @@ RUN curl -sSL ${PS_PACKAGE_URL} -o /tmp/powershell.rpm \
       gssntlmssp \
       # less is required for help in powershell
       less \
+      # clear is part of ncurses which is used by clear
+      ncurses \
     && yum upgrade-minimal -y --security \
     && yum clean all \
     # remove powershell package