[databases] Use current user ID if none is given when indexing

This will generate files for the current user like all other
files generated when using beat/beat.cmdline>

Fixes #78

beat/cmdline/databases.py

@@ -177,8 +177,10 @@ def start_db_container(
 
     db_configuration = {"inputs": {}, "channel": set_name}
 
-    if uid is not None:
-        db_configuration["datasets_uid"] = uid
+    if uid is None:
+        uid = os.getuid()
+
+    db_configuration["datasets_uid"] = uid
 
     if db_root is not None:
         db_configuration["datasets_root_path"] = db_root