Distill to single Dockerfile

[hairyhenderson]

This does a few things:

• removes the templated Dockerfiles (hardcoding makes it simpler for Renovate to automatically pick up base image updates)
• brings the 2 (alpine and scratch) variants together into a single Dockerfile (just build with --target to differentiate)
• moves the Dockerfile to the root of the repo to make the docker build command a bit simpler
• adds some standard OCI labels (Fixes Add LABELS instructions #4)
• adds an entrypoint script to the alpine variant so that non-caddy commands can be run more easily
• sets up some custom build-phase hook scripts to run in the (not-yet-set-up) Automated Build to be set up in DockerHub
• adds a simple GitHub Action to make sure the images build successfully

This doesn't:

• update the Taskfile.yml - I'm not familiar with that so much so I'll need to defer to @Ilyes512 on that (or we could drop that entirely since the hook/build script handles the build)
• do anything that's covered by WIP: Create and use caddy user instead of root #6 - hopefully it's not too difficult to merge those changes in as part of a follow-up PR

Signed-off-by: Dave Henderson [email protected]

[hairyhenderson]

The Action ran, but because this came from my fork it ran in my fork: https://github.com/hairyhenderson/caddy-docker/commit/9045ec2fa1b015ea1baf17feb9713edddf922bcc/checks?check_suite_id=287357456 (tl;dr: it passes 😅)

[hairyhenderson]

FYI here's the latest (passing) Actions run from my fork: https://github.com/hairyhenderson/caddy-docker/commit/4756e9bf2b4ea2f71cc0b792310132dba4356caf/checks?check_suite_id=292066853