makefiles/docker.inc.mk: add a 'docker_volume' function
makefiles/docker.inc.mk: add a 'docker_volume' function

Refactor the '--volume' mounting handling.
This prepares for using a global 'DOCKER_VOLUME_OPTIONS'.

yogo1212/RIOT,basilfx/RIOT,kYc0o/RIOT,mtausig/RIOT,kaspar030/RIOT,toonst/RIOT,authmillenon/RIOT,toonst/RIOT,ant9000/RIOT,toonst/RIOT,kYc0o/RIOT,OlegHahm/RIOT,josephnoir/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,yogo1212/RIOT,x3ro/RIOT,miri64/RIOT,smlng/RIOT,OlegHahm/RIOT,ant9000/RIOT,basilfx/RIOT,rfuentess/RIOT,OTAkeys/RIOT,smlng/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,miri64/RIOT,x3ro/RIOT,jasonatran/RIOT,mtausig/RIOT,mtausig/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,rfuentess/RIOT,basilfx/RIOT,yogo1212/RIOT,mtausig/RIOT,OlegHahm/RIOT,yogo1212/RIOT,kaspar030/RIOT,aeneby/RIOT,ant9000/RIOT,x3ro/RIOT,smlng/RIOT,smlng/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,ant9000/RIOT,OTAkeys/RIOT,kaspar030/RIOT,yogo1212/RIOT,jasonatran/RIOT,ant9000/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,x3ro/RIOT,kYc0o/RIOT,kaspar030/RIOT,basilfx/RIOT,toonst/RIOT,OlegHahm/RIOT,jasonatran/RIOT,smlng/RIOT,aeneby/RIOT,OTAkeys/RIOT,toonst/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,jasonatran/RIOT,miri64/RIOT,x3ro/RIOT,josephnoir/RIOT,rfuentess/RIOT,mtausig/RIOT,kaspar030/RIOT,authmillenon/RIOT,aeneby/RIOT,miri64/RIOT,aeneby/RIOT,OlegHahm/RIOT,josephnoir/RIOT,aeneby/RIOT,basilfx/RIOT,rfuentess/RIOT,miri64/RIOT