[tempo-distributed] tempo-gateway should only mount config file

[boniek83]

Currently:

         volumeMounts:
            - name: config
              mountPath: /etc/nginx

should be:

         volumeMounts:
             - mountPath: /etc/nginx/nginx.conf
               name: config
               subPath: nginx.conf

Why? Because otherwise I get the following:
/docker-entrypoint.sh: No files found in /docker-entrypoint.d/, skipping configuration
2025/05/12 09:23:22 [crit] 1#1: pread() "/etc/nginx/nginx.conf" failed (21: Is a directory)
nginx: [crit] pread() "/etc/nginx/nginx.conf" failed (21: Is a directory)