Update session cookie key name to be unique to instance in development

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/31644

changelogs/unreleased/31644-make-cookie-sessions-unique.yml

+---
+title: Update session cookie key name to be unique to instance in development
+merge_request:
+author:

config/initializers/session_store.rb

@@ -10,6 +10,12 @@ rescue
   Settings.gitlab['session_expire_delay'] ||= 10080
 end
+cookie_key = if Rails.env.development?
+               "_gitlab_session_#{Digest::SHA256.hexdigest(Rails.root.to_s)}"
+             else
+               "_gitlab_session"
+             end
+
 if Rails.env.test?
   Gitlab::Application.config.session_store :cookie_store, key: "_gitlab_session"
 else
@@ -19,7 +25,7 @@ else
   Gitlab::Application.config.session_store(
     :redis_store, # Using the cookie_store would enable session replay attacks.
     servers: redis_config,
-    key: '_gitlab_session',
+    key: cookie_key,
     secure: Gitlab.config.gitlab.https,
     httponly: true,
     expires_in: Settings.gitlab['session_expire_delay'] * 60,