Updated ostree in Fedora 25

I believe that Fedora has pushed a new ostree (2017.8-2.fc25) that has the commits (9a79d13ce307d8b5b2fe0c373c5d778ad7128b5a) referred to in Dockerfile-build.sh.

For FC25, see https://bodhi.fedoraproject.org/updates/FEDORA-2017-efabd0d782

(We could also alter Dockerfile to use FC26 too?)

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