makefiles/docker.inc.mk: add DOCKER_APPDIR variable
makefiles/docker.inc.mk: add DOCKER_APPDIR variable

Prepare for when it can have a different value.

ant9000/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,smlng/RIOT,A-Paul/RIOT,basilfx/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,cladmi/RIOT,x3ro/RIOT,yogo1212/RIOT,OlegHahm/RIOT,mtausig/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,mtausig/RIOT,x3ro/RIOT,miri64/RIOT,lazytech-org/RIOT,mfrey/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kaspar030/RIOT,x3ro/RIOT,OlegHahm/RIOT,A-Paul/RIOT,authmillenon/RIOT,aeneby/RIOT,OTAkeys/RIOT,aeneby/RIOT,yogo1212/RIOT,jasonatran/RIOT,mfrey/RIOT,jasonatran/RIOT,kYc0o/RIOT,josephnoir/RIOT,rfuentess/RIOT,aeneby/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,toonst/RIOT,lazytech-org/RIOT,rfuentess/RIOT,OTAkeys/RIOT,cladmi/RIOT,jasonatran/RIOT,mfrey/RIOT,ant9000/RIOT,yogo1212/RIOT,lazytech-org/RIOT,ant9000/RIOT,mtausig/RIOT,basilfx/RIOT,yogo1212/RIOT,josephnoir/RIOT,authmillenon/RIOT,basilfx/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,mtausig/RIOT,rfuentess/RIOT,ant9000/RIOT,authmillenon/RIOT,rfuentess/RIOT,miri64/RIOT,mfrey/RIOT,cladmi/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,toonst/RIOT,aeneby/RIOT,lazytech-org/RIOT,smlng/RIOT,cladmi/RIOT,mfrey/RIOT,OlegHahm/RIOT,toonst/RIOT,jasonatran/RIOT,kaspar030/RIOT,x3ro/RIOT,toonst/RIOT,A-Paul/RIOT,basilfx/RIOT,josephnoir/RIOT,smlng/RIOT,cladmi/RIOT,aeneby/RIOT,mtausig/RIOT,yogo1212/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,smlng/RIOT,smlng/RIOT,toonst/RIOT,kaspar030/RIOT,x3ro/RIOT,miri64/RIOT,kaspar030/RIOT,lazytech-org/RIOT,A-Paul/RIOT,A-Paul/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT