Add the tag.sh script to this repository

Since the GitHub Actions configuration builds images on tag pushes, we should
use the standard tagging script we use for the same purpose in other
repositories. This will make it easier to tag, build, and push Docker images
of a specific version in a standardized manner.

tag.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+set -o nounset
+set -o errexit
+set -o pipefail
+
+version=$(./bump_version.sh show)
+
+git tag "v$version" && git push --tags