Fix sha256 format for CI PostgreSQL image

Author: JJ-8

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     services:
       postgres:
-        image: postgres:596e4c843a9db32269a3757624d8a6a6f633e01895acb83fe0842497fd897eb7
+        image: postgres@sha256:596e4c843a9db32269a3757624d8a6a6f633e01895acb83fe0842497fd897eb7
         env:
           POSTGRES_PASSWORD: ctfnote
           POSTGRES_USER: ctfnote