chore: pin shas in Dockerfile

Dockerfile

@@ -1,6 +1,6 @@
 #checkov:skip=CKV_DOCKER_2
 #checkov:skip=CKV_DOCKER_3
-FROM python:3.12-slim
+FROM python:3.12-slim@sha256:e3ae8cf03c4f0abbfef13a8147478a7cd92798a94fa729a36a185d9106cbae32
 LABEL com.github.actions.name="contributors" \
     com.github.actions.description="GitHub Action that given an organization or repository, produces information about the contributors over the specified time period." \
     com.github.actions.icon="users" \