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)

```console
$ docker run -it --rm 3950d8b65fad locale
LANG=C.UTF-8
LANGUAGE=
LC_CTYPE="C.UTF-8"
LC_NUMERIC="C.UTF-8"
LC_TIME="C.UTF-8"
LC_COLLATE="C.UTF-8"
LC_MONETARY="C.UTF-8"
LC_MESSAGES="C.UTF-8"
LC_PAPER="C.UTF-8"
LC_NAME="C.UTF-8"
LC_ADDRESS="C.UTF-8"
LC_TELEPHONE="C.UTF-8"
LC_MEASUREMENT="C.UTF-8"
LC_IDENTIFICATION="C.UTF-8"
LC_ALL=
```

So, unless Haskell actually uses `LC_ALL` or `LANGUAGE` directly (which it really shouldn't be doing), this should be sufficient.