[python/Dockerfile] Put fontbakery into /var/fontbakery instead of /tmp
[python/Dockerfile] Put fontbakery into /var/fontbakery instead of /tmp

googlefonts/fontbakery-dashboard,googlefonts/fontbakery-dashboard,googlefonts/fontbakery-dashboard,googlefonts/fontbakery-dashboard,googlefonts/fontbakery-dashboard