document compatibility workarounds

compose.yaml

@@ -25,6 +25,8 @@ services:
     command: php artisan serve --host=0.0.0.0 --port=8000
 
   db:
+    # TODO(compat): Upgrade MySQL 5.7 after validating a restored data copy,
+    # migrations and reports. Update db-test to the same target version.
     image: mysql:5.7
     environment:
       MYSQL_DATABASE: pollux
@@ -42,6 +44,8 @@ services:
       retries: 20
 
   db-test:
+    # TODO(compat): Replace this legacy image alongside db during the MySQL
+    # upgrade; keep the test database isolated and run the full test suite.
     image: mysql:5.7
     profiles: [testing]
     environment: