Better (and automatic) version info embedding

[natsukagami]

We currently rely on changing essentially doc.go to display the production version and compare with the latest version from GitHub to look for updates:

kjudge/doc.go

Lines 5 to 10 in 6dec3df

	// The version and codename of the system.
	// We typically pick an anime JK name as the codename for every minor version.
	const (
	Version = "0.7.4"
	Codename = "Maaya Awatsuki"
	)

This requires a manual commit for every release (as I have forgotten to do in 0.7.5). Let's look for a way to automate this.

Perhaps something like debug.BuildInfo or https://github.com/docker/metadata-action (to extract and inject metadata during CI builds) would help.