$ sudo apt-get update
$ sudo apt-get install qgis-mapserver 

qgis-mapserver is installed in your system.

Make ensure the qgis-mapserver package were installed using the commands given below,

$ sudo dpkg-query -l | grep qgis-mapserver *

You will get with qgis-mapserver package name, version, architecture and description in a table

qgis-mapserver package basic information: