docs: improve Dockerfile for hardened images (#2270)
Prevents errors like this one when using Mercure: `php-1 | Error:
loading initial config: loading new config: loading frankenphp app
module: provision frankenphp: failed to provision caddy http: loading
http app module: provision http: server srv0: setting up route handlers:
route 2: loading handler modules: position 2: loading module 'mercure':
provision http.handlers.mercure: provision http.handlers.mercure.bolt:
"": invalid transport: open /data/caddy/mercure.db: permission denied`
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Signed-off-by: Kévin Dunglas <kevin@dunglas.fr>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
