nix: improve prymn_db script, adding a 'shell' command

nix/scripts.nix

@@ -15,6 +15,7 @@
       export PGHOST=/tmp
       export DB_LOG=$PROJECT_ROOT_DIR/.db/log
 
+      start_db() {
         if [ ! -d "$PGDATA" ]; then
             initdb "$PGDATA" --auth=trust
         fi
@@ -28,6 +29,19 @@
         if [ "$user_exists" != "1" ]; then
             createuser -s -h "$PGHOST" postgres
         fi
+      }
+
+      command="''${1-default}"
+      case $command in
+        shell)
+          psql -U postgres
+          exit 0
+          ;;
+        start | default | *)
+          start_db
+          exit 0
+          ;;
+      esac
     '';
   };
 }