Update docker-nightly-build.yml

Remove other version than 15.

Author: ChakshuGautam

.github/workflows/docker-nightly-build.yml

@@ -24,7 +24,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        pg: [12, 13, 14, 15]
+        pg: [15]
 
     steps:
     - uses: actions/checkout@v3