upload_artifacts.sh is hard to manually test

Currently it can only be run on Kokoro and only for tagged releases. @zpencer, has a kokoro job that lets him kick off a job against a private (fake) tagged release but this doesn't scale to more users and is slow as it requires building all dependent artifacts.

We need a way to be able to run the majority of the script from our workstations. This is generally not too hard with a little bit of reorganization, with the exception of gpg. While with gnupg 2.x we could leverage the gpg-agent to split kokoro-specific code out, there's not an easy way for gnupg 1.x. We either need to use something similar to the release key for testing or require gnupg 2.x. Since Kokoro is using Ubuntu 14.04 which has gnupg 1.x, we could use a docker container to get gnupg 2.x.