perf: use ChainMap in resolve_variables to avoid redundant dict copies

Author: RailtonDantas

src/dotenv/main.py

@@ -7,6 +7,7 @@
 import sys
 import tempfile
 from collections import OrderedDict
+from collections import ChainMap
 from contextlib import contextmanager
 from typing import IO, Dict, Iterable, Iterator, Mapping, Optional, Tuple, Union
 
@@ -247,18 +248,16 @@ def resolve_variables(
 ) -> Mapping[str, Optional[str]]:
     new_values: Dict[str, Optional[str]] = {}
 
+    if override:
+        env = ChainMap(new_values,os.environ)
+    else:
+        env = ChainMap(os.environ,new_values)
+
     for name, value in values:
         if value is None:
             result = None
         else:
             atoms = parse_variables(value)
-            env: Dict[str, Optional[str]] = {}
-            if override:
-                env.update(os.environ)  # type: ignore
-                env.update(new_values)
-            else:
-                env.update(new_values)
-                env.update(os.environ)  # type: ignore
             result = "".join(atom.resolve(env) for atom in atoms)
 
         new_values[name] = result