docker-compose: add volume mapping for database persistence

.gitignore

@@ -9,4 +9,5 @@ worktrees/
 htmlcov/
 creds.txt
 .tmp_mig.db
-.tmp_pytest
+.tmp_pytest
+db/

docker-compose.yml

@@ -9,6 +9,8 @@ services:
       MARIADB_ROOT_PASSWORD: rootpass
     ports:
       - "3306:3306"
+    volumes:
+      - ./db:/var/lib/mysql
     healthcheck:
       test: ["CMD", "mariadb-admin", "ping", "-h", "localhost", "-uroot", "-prootpass"]
       interval: 5s