chore(server): document and maintain (#42)

This commit makes sure we thoroughly document how the server works and
applies some maintenance changes.

The most relevant changes are:

1. using io.ReadAll instead of ioutil.ReadAll;

2. dropping rand.Seed calls.

We can safely drop rand.Seed calls because:

> Deprecated: As of Go 1.20 there is no reason to call Seed
> with a random value. Programs that call Seed with a known value
> to get a specific sequence of results should use
> New(NewSource(seed)) to obtain a local random generator.

We use go1.20 in Dockerfile and go.mod.