utils: Fix datetime_to_timestamp

Replace usage of deprecated function `datetime.utcfromtimestamp` and
make sure the input date is UTC before subtracting.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Author: vvoland

docker/utils/utils.py

@@ -5,7 +5,7 @@
 import os.path
 import shlex
 import string
-from datetime import datetime
+from datetime import datetime, timezone
 from packaging.version import Version
 
 from .. import errors
@@ -394,8 +394,8 @@ def convert_filters(filters):
 
 
 def datetime_to_timestamp(dt):
-    """Convert a UTC datetime to a Unix timestamp"""
-    delta = dt - datetime.utcfromtimestamp(0)
+    """Convert a datetime to a Unix timestamp"""
+    delta = dt.astimezone(timezone.utc) - datetime(1970, 1, 1, tzinfo=timezone.utc)
     return delta.seconds + delta.days * 24 * 3600