Allow su-exec to fail when users explicity use --user

This allows MinIO containers to run properly without
expecting higher privileges in situations where following
restrictions on containers are used

 - docker run --user uid:gid
 - docker-compose up (with docker-compose.yml with user)
 ```yml
 ...
 user: "1001:1001"
 command: minio server /data
 ...
 ```
 - All openshift containers

Fixes minio#7773

dockerscripts/docker-entrypoint.sh

@@ -60,7 +60,12 @@ docker_switch_user() {
             return
         fi
     fi
-    exec su-exec "${owner}" "$@"
+    # check if su-exec is allowed, if yes proceed proceed.
+    if su-exec "${owner}" "/bin/ls" >/dev/null 2>&1; then
+        exec su-exec "${owner}" "$@"
+    fi
+    # fallback
+    exec "$@"
 }
 
 ## Set access env from secrets if necessary.