ENV to override the server host

[jwoertink]

Purpose

Fixes #1787

Description

This adds the option to override the server host by using an ENV. We've had the ability to do this for the PORT for a while now, so it made sense to add host too.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test