Share the self-signed Caddy CA/cert across worktrees (#211)

Each worktree previously bind-mounted its own frankenphp/data, so
Caddy minted a fresh CA and cert per checkout and just trust-cert had
to be re-run every time. Anchor CADDY_DATA_DIR at the shared .git
common dir instead, so all worktrees of the same clone reuse one CA.

compose.override.yaml

@@ -10,7 +10,7 @@ services:
       - ~/.composer/cache:/root/.composer/cache
       - ./frankenphp/Caddyfile:/etc/caddy/Caddyfile:ro
       - ./frankenphp/conf.d/20-app.dev.ini:/usr/local/etc/php/app.conf.d/20-app.dev.ini:ro
-      - ./frankenphp/data:/data
+      - ${CADDY_DATA_DIR:-./frankenphp/data}:/data
       - sass:/app/var/sass
     environment:
       MERCURE_EXTRA_DIRECTIVES: demo