Honour http_proxy env variable when fetching gpg keys

[hbog]

http_proxy can be defined as build argument when building the
container behind a proxy.
Since version 2.1 of GnuPG, dirmngr takes care of accessing keyservers,
but dirnmgr does not honor http_proxy unless it is configured to do so.
Therefore building the container behind a firewall fails since the base image
was switched from debian/jessie to debian/stretch.

[tianon]

I'm curious why you're rebuilding instead of simply using the pre-built artifacts; can you explain your use case a little bit?

[hbog]

I have built customized base images that contain a few location specific settings, such as our locale. Hence location specific image extensions are done only once in a base image, rather then writing an extension for every type of container. This generates localized images using universal, portable build scripts. I submitted a PR because I can image that there might be other reasons for not using the pre-built artifacts.

[tianon]

Closing given none of our build environments use a proxy in such an explicit way. There are also some notes in https://github.com/docker-library/faq#openpgp--gnupg-keys-and-verification which could be used to accomplish this without too much overhead (hijacking container DNS to point to a proxy-aware reverse proxy for these requests).

For what it's worth, things like added locales would more easily be handled via something like:

FROM postgres:xxx
RUN localedef -i de_DE -c -f UTF-8 -A /usr/share/locale/locale.alias de_DE.UTF-8
ENV LANG de_DE.utf8

(as noted under "Locale Customization" on https://hub.docker.com/_/postgres)