makefiles/docker.inc.mk: handle building in git worktree
makefiles/docker.inc.mk: handle building in git worktree

When building from a worktree, the common git directory was not mounted
in docker.

This lead to the version not being set and issues with git-cache in
ubuntu bionic that could not execute the 'git hash-object' command.

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