Use TLS to fetch nginx sources, signatures and public keys

[J0WI]

No description provided.

[thresheek]

We're checking sources against a known signature, so TLS is absolutely unneeded here.

Also, at least keyserver.ubuntu.com does not speak hkps on port 80 - did you actually check what you've committed?

[J0WI]

There was already discussion here about the benefits of TLS. In the case of http I see no drawback.
Meanwhile, using https has become a common practice for official images:

docker-library/golang#12
docker-library/drupal#55
docker-library/ruby#87
docker-library/redmine#34
docker-library/php#293
docker-library/busybox@8b0d966
docker-library/memcached@0991958

I tested the build locally and it works for me. If you prefer, I can split the hkps change into another PR. I never seen Dockerfiles using multiple key servers, but I also didn't found a guideline about this topic.

[thresheek]

It works because it uses the first gpg keyserver to fetch the keys. It does indeed seems that https is becoming prevalent in official images, so fix that PR and re-send.

[J0WI]

@thresheek I've created #208.