fix: update regex in airflowignore to correctly ignore hidden files and directories

Signed-off-by: Max Ostapenko <1611259+max-ostapenko@users.noreply.github.com>

Author: max-ostapenko

airflow/dags/.airflowignore

@@ -1,6 +1,6 @@
 # Ignore common helpers, utilities, and tests from DAG processing (regex syntax)
 common/.*
-\..*
+(^|/)\..*
 __pycache__/.*
 .*\.pyc$