feat: pass through SERVER_IGNORE_HIDDEN_FILES env

static-web-server skips dot-prefixed paths by default. Passing this
env through lets operators set SERVER_IGNORE_HIDDEN_FILES=false to
serve e.g. /.well-known/matrix/server for Matrix federation
delegation.

Backwards-compatible: unset means existing behaviour (hidden files
skipped).

compose.yml

@@ -20,6 +20,7 @@ services:
         - "coop-cloud.${STACK_NAME}.version=1.0.1+2.38.0"
     environment:
       - HTTP_SUBDIR
+      - SERVER_IGNORE_HIDDEN_FILES
     volumes:
       - content:/var/www/http