Add `--fix-missing` to trusty's Dockerfile

When adopting dox on Zaqar and Glance, the ubuntu image required running
the update command with `--fix-missing`. I'm adding it directly to the
Dockerfile since it seems harmless, AFAICT.

Change-Id: I126ed9c4416924cce44ee3e4498e21e11157fe2a
Merge "Add `--fix-missing` to trusty's Dockerfile"