Make PostgreSQL quality cleanup reliable

tools/quality/quality.sh

@@ -208,7 +208,7 @@ run_test_suite() {
   )
 }
 
-run_postgresql_migration_check() {
+run_postgresql_migration_check() (
   local python container port
   python="$(python_command)"
   require_command docker
@@ -225,7 +225,10 @@ run_postgresql_migration_check() {
     --env POSTGRES_USER=postgres \
     --env POSTGRES_PASSWORD=quality \
     --publish 127.0.0.1::5432 postgres:17-alpine >/dev/null
-  port="$(docker port "${container}" 5432/tcp | sed 's/.*://')"
+  port="$(docker port "${container}" 5432/tcp)"
+  port="${port%%$'\n'*}"
+  port="${port##*:}"
+  [[ ${port} =~ ^[0-9]+$ ]]
 
   for _attempt in {1..30}; do
     if docker exec "${container}" pg_isready \
@@ -251,7 +254,7 @@ run_postgresql_migration_check() {
   )
   cleanup_postgres
   trap - EXIT HUP INT TERM
-}
+)
 
 run_release_lint() {
   local python