docker: Use system GIT_CACHE_DIR if available
docker: Use system GIT_CACHE_DIR if available

If GIT_CACHE_DIR is a directory make it available to docker.
This will allow using the system git_cache also in the docker container.

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