Add a few minor tweaks

- use full GPG fingerprint to fetch GPG key (this way, we not only fetch the key, but verify the full fingerprint too, and the build will actually fail outright if the machine doing the building has been MITM'd)
- remove `DEBIAN_FRONTEND` since it's just for cosmetics during build (no reason to add a full extra layer just for that)
- remove extra locale-related env vars (setting `LANG` alone should be enough, as verified by running `locale` once inside the resulting container)
Merge pull request #9 from tianon/minor-tweaks

Add a few minor tweaks