Adds Optional SQLite Docker

[therealkevinard]

This PR allows opt-in SQLite support with different build/run commands.

usage:

• build checkup:sqlite3 with make docker_sqlite
• start stack with docker-compose -f docker-compose.sqlite3.yml

The d/c stack includes a separate worker, allowing its own entrypoint directive. eg: checkup every 10s, independent of the frontend.
A sqlite config.js is bind-mounted from ./statuspage/js/config_sqlite3.js

The built docker image is larger: 155MB vs the fs-only image's 33.4 MB.

This is to address #140

[titpetric]

Hey, thanks for the contribution. Could you please just squash the commits so I have a better overview of the proposed change(s) over the multiple commits.

[therealkevinard]

Of course! :+1: