makefiles/docker.inc.mk: use BUILD_DIR in RIOT if possible
makefiles/docker.inc.mk: use BUILD_DIR in RIOT if possible

Use BUILD_DIR from within the riot repository if it is inside.

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