Update postgres

Changes:

- docker-library/postgres@3690694: Merge pull request docker-library/postgres#797 from infosiftr/rc-beta-alpha
- docker-library/postgres@dba8ec0: Adjust tagging to enforce explicit pre-release opt-in

library/postgres

@@ -1,4 +1,4 @@
-# this file is generated via https://github.com/docker-library/postgres/blob/9abfeee61650bc84d181f46293b06ef29934dd50/generate-stackbrew-library.sh
+# this file is generated via https://github.com/docker-library/postgres/blob/dba8ec0bf97b220682d2cfe417e5d4f367df8a92/generate-stackbrew-library.sh
 
 Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)