Fix exec_sql to use configured credentials instead of hardcoded defaults

The exec_sql function hardcoded PGPASSWORD=password and -U postgres
when connecting via psql. This caused init scripts in
/docker-entrypoint-initdb.d/ to silently fail when DOLTGRES_USER or
DOLTGRES_PASSWORD were set to custom values, because the postgres
user no longer exists.

Use the existing get_env_var helper to read DOLTGRES_USER/PASSWORD
(or POSTGRES_USER/PASSWORD), falling back to postgres/password only
when neither is set.

Fixes #2300

Author: aegis/crew/arnold

docker-entrypoint.sh

@@ -92,15 +92,21 @@ exec_sql() {
   echo "running psql command found in: "
   echo $(which psql)
 
+  local pg_user pg_password
+  pg_user=$(get_env_var "USER")
+  pg_password=$(get_env_var "PASSWORD")
+  pg_user="${pg_user:-postgres}"
+  pg_password="${pg_password:-password}"
+
   while true; do
       if [ -n "$query" ]; then
           set +e
-          output=$(PGPASSWORD=password psql -h 127.0.0.1 -U postgres -c "$query" 2>&1)
+          output=$(PGPASSWORD="$pg_password" psql -h 127.0.0.1 -U "$pg_user" -c "$query" 2>&1)
           status=$?
           set -e
     else
       set +e # tmp disabled to initdb.d/ file err
-      output=$(PGPASSWORD=password psql -h 127.0.0.1 -U postgres < /dev/stdin 2>&1)
+      output=$(PGPASSWORD="$pg_password" psql -h 127.0.0.1 -U "$pg_user" < /dev/stdin 2>&1)
       status=$?
       set -e
     fi