infra/image/shcontainer: Ensure '/ect/shadow' is readable

The shadow file in a Fedora or CentOS Stream container is not readable
any more using Ubuntu 24.04.

An extra call to ensure that the shadow file is readable again has been
added to container_start.

infra/image/shcontainer

@@ -47,6 +47,8 @@ container_start() {
 
     log info "= Starting ${name} ="
     podman start "${name}"
+    # Ensure /etc/shadow is readable
+    podman exec "${name}" bash -c "chmod u+r /etc/shadow"
     echo
 }