ci: fix sudo in build image (cvmfs#3816)

Rebuilding the build images caused sudo to fail, with an error
reminiscent of apptainer/apptainer#2756
(although gh uses docker):
```
 [100%] Built target auto.cvmfs
sudo: PAM account management error: Authentication service cannot retrieve authentication info

sudo: a password is required
```
The chmod fixes this issue, although it's not yet clear to me why.

Author: vvolkl

test/common/container/Dockerfile-dev

@@ -2,6 +2,10 @@ FROM gitlab-registry.cern.ch/cernvm/build-images/centos_x86_64:9
 
 USER root
 
+# This is a workaround for a sudo error that started after an 
+# upgrade to the build image. Similar to https://github.com/apptainer/apptainer/issues/2756 but not quite clear
+RUN chmod 0400 /etc/shadow
+
 RUN dnf install -y sudo autofs fuse fuse3 httpd wget lsof \
     python python3-pip attr nc sqlite python3-mod_wsgi jq usbutils