Fix issue with data volume mounted on host

When mounting /opt/graphite/storage as data
volume on the host, the previous syncdb will
be lost. So instead of doing syncdb in the
build command, we should call syncdb only
the first time the image is run.

This fixes #11 and #12
Merge pull request #15 from samos123/fix-host-data-volume-issue

Fix issue with data volume mounted on host