Configure Docker Compose through .env for Arcane

docker-compose.yml now reads every host-specific value (paths, user,
port, network, time zone) from .env with the previous values as
defaults. .env.example documents them; .env stays out of Git.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -3,3 +3,5 @@ web/node_modules
 web/dist
 config
 *.db
+.env
+.env.*