Goreleaser

[tsmethurst]

This PR reworks some of the automation and build steps for GtS.

In particular, it adds a dependency on goreleaser, which we can use for easily producing multi-architecture builds, and packaging release files/tars/etc etc.

In order to make goreleaser work in our existing Drone ci/cd pipeline, I also had to make a few changes to the drone file.

The overall goal of this PR is to allow the following to happen:

1. When a merge is done into main from a pull request, build a docker image snapshot using goreleaser, and then push it as latest (so, similar to what we do now).
2. When a tag is pushed to the repository, cut a release from the tag, build all the binaries and tar them up with assets, and then push them onto github. ALSO push a docker container with that tag to the docker repo, and update 'latest'. In other words, totally automate the release when a tag is pushed.