Update "gpg" invocations to use "--batch"

[tianon]

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

[yosifkit]

LGTM

[yosifkit]

See also:

• https://github.com/docker-library/cassandra/pull/170
• https://github.com/docker-library/elasticsearch/pull/185
• https://github.com/docker-library/gcc/pull/50
• https://github.com/docker-library/ghost/pull/164
• https://github.com/docker-library/httpd/pull/117
• https://github.com/docker-library/julia/pull/26
• https://github.com/docker-library/kibana/pull/88
• https://github.com/docker-library/logstash/pull/94
• https://github.com/docker-library/mariadb/pull/212
• https://github.com/docker-library/mongo/pull/314
• https://github.com/docker-library/mysql/pull/515
• https://github.com/docker-library/owncloud/pull/108
• https://github.com/docker-library/percona/pull/73
• https://github.com/docker-library/php/pull/747
• https://github.com/docker-library/postgres/pull/527
• https://github.com/docker-library/python/pull/351
• https://github.com/docker-library/rabbitmq/pull/289
• https://github.com/docker-library/redis/pull/171
• https://github.com/docker-library/redmine/pull/140
• https://github.com/docker-library/tomcat/pull/137
• https://github.com/docker-library/wordpress/pull/348
• https://github.com/tianon/docker-bash/pull/14

[tianon]

• https://github.com/jessfraz/irssi/pull/21
• https://github.com/ChristophBerg/pgdg-keyring/pull/2
• https://github.com/boot2docker/boot2docker/commit/9f98402e63ac88db82f63de58221228407f7b9ed
• https://github.com/neurodebian/dockerfiles/pull/12