[NTG-context] Docker images

Hi, esp. Max: the ConTeXt dockerfiles from “Island of TeX” (https://github.com/islandoftex/context) … – still distinguish MkIV beta/current – install modules via rsync also for LMTX which pulls in some outdated ones – use Debian instead of Alpine because Linux/MUSL binary wasn’t working at the time (but should now, otherwise compiling it on the fly is an option) Would it make sense to change this? Hraban