test: add podman/docker dbtest tool for postgres, mssql and mysql

tests/dbtest/init/mysql.sql

@@ -0,0 +1,21 @@
+-- Minimal MySQL fixture for relspec container tests.
+-- Extend when a MySQL reader/writer is added.
+CREATE TABLE users (
+    id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+    email VARCHAR(255) NOT NULL,
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE KEY uq_users_email (email)
+) ENGINE=InnoDB;
+
+CREATE TABLE posts (
+    id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+    user_id BIGINT UNSIGNED NOT NULL,
+    title VARCHAR(255) NOT NULL,
+    body TEXT NULL,
+    KEY idx_posts_user (user_id),
+    CONSTRAINT fk_posts_user FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE CASCADE
+) ENGINE=InnoDB;
+
+CREATE VIEW v_user_posts AS
+    SELECT u.id AS user_id, u.email, p.id AS post_id, p.title
+    FROM users u JOIN posts p ON p.user_id = u.id;