Add alternatives to gpg server

[bhasterix]

When executing the build it stalled intermittently on the gpg request for keys, adding alternatives decreases the cnahce of this happening.

[yosifkit]

Copying my comment from docker-library/cassandra#131 (comment)

See also, docker-library/mysql#263 (comment) and docker-library/httpd#66 (comment)

We've gone back and forth on adding a similar loop but don't want to yet open the flood gates to repeat this loop in all of the images maintained in http://github.com/docker-library. We recommend just pulling the images from Docker Hub rather than building them yourself, but if you must build it, then editing the file to choose a gpg server that is better for your location is the solution (a quick sed or awk before docker build).

[bhasterix]

I'm in a situation where we have to build our own, so the sed option is how we're going now.
I thought I'd submit the pull request to do it for all but point taken on not impacting all like this, thanks.

What about looking at a common utilities library that all repos could pull from? Then changing this will be a pain but changing anything similar in future would be relatively easy.

[yosifkit]

We've thought about making a set of scripts for downloading gpg keys reliably but have moved toward solving this on the server side to have a proxy that duplicates the gpg request to mutliple gpg servers and responds with the fastest good response.