Feature / experiment: develop an alternative frontend container using 'caddy' as a webserver

[jayaddison]

Following discussion in #183, self-signed TLS certificates have been deprecated and removed from the frontend container image.

While looking into alternative approaches that can provide on-demand TLS certificate provisioning -- something that could be useful in both public-facing and internal environments -- caddy seemed worthwhile to investigate since it supports this functionality natively.

It could be worth exploring caddy as an alternative frontend webserver (replacing nginx).

[jayaddison]

A counterpoint / reason to perhaps discourage switching to caddy: it looks much less widely-used than nginx -- most recent stats from Nov 2022 indicate that nginx is the world's most popular webserver by quite a large margin (34%+ market share) - https://w3techs.com/technologies/overview/web_server Caddy is a recent entrant and is likely growing at a relatively fast rate, although it currently has ~0.1% market share.

Even so: I think the on-demand TLS with a self-hosted local CA is a compelling feature and worth exploring.

[jayaddison]

Closing as done; pull request #192 contains a fairly thorough experimental migration to caddy that could be picked up again in future.