Fix "gpg" usage to include "--batch"

[tianon]

See https://bugs.debian.org/913614, especially:

PS i do encourage everyone who is automating the use of gpg to use --batch everywhere, as this forces GnuPG into a mode that is expected to be used for automation (its "API", for lack of a better term, as opposed to its "UI", which is its normal non-batch mode).

Fixes #41
Refs docker-library/official-images#5082

See also docker-library/busybox#55 and linked issues/PRs.

[keeganwitt]

I think we might want --no-tty too. The man page says

--no-tty
Make sure that the TTY (terminal) is never used for any output.
This option is needed in some cases because GnuPG sometimes
prints warnings to the TTY even if --batch is used.

The error I saw previously that made me add it in the first place was "gpg: cannot open '/dev/tty': No such device or address" I believe.

[keeganwitt]

Build is still flaky, even with both args. Last error was "gpg: keyserver receive failed: Cannot assign requested address"

[tianon]

Indeed, this PR is not expected to fix any flakiness; this is simply a GnuPG usage fix.

For flakiness suggestions, see docker-library/official-images#4252, especially docker-library/php#666 and linked builds.