makefiles/docker.inc.mk: use 'docker_volume' function
makefiles/docker.inc.mk: use 'docker_volume' function

Use 'docker_volume' function for mounting.

When mounting 'localtime' this will only use the 'ro' option and
ignore 'DOCKER_VOLUME_OPTIONS'.

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