feat: verify cosign image signatures before pulling ghcr.io/nextcloud-releases images

Agent-Logs-Url: https://github.com/nextcloud/all-in-one/sessions/fd95dc7d-c221-4c10-9e14-e6f2253a7a22

Co-authored-by: szaimen <42591237+szaimen@users.noreply.github.com>

Containers/mastercontainer/Dockerfile

@@ -46,7 +46,8 @@ RUN set -ex; \
         sudo \
         netcat-openbsd \
         curl \
-        grep; \
+        grep \
+        cosign; \
     \
     apk add --no-cache --virtual .build-deps \
         autoconf \