Default Java heap for headless-server reset to 1G in Dockerfile

This change only affects customers coming from CoreMedia versions 2101.1, 2101.2 and 2101.3 and use the Docker deployment. The default Java heap for the Headless Server has been reset from 2048m to 1024m in the Dockerfile of the Headless Server. The Java heap can, but does not have to be increased for performance optimization.

(CMS-19210)