Test all supported databases in CI (#2000)

* add mysql and postgres tests to CI

docker-testing/docker-compose-postgres.yml

@@ -0,0 +1,23 @@
+version: '2.4'
+services:
+  postgres:
+    image: "postgres:10"
+    restart: always
+    environment:
+      POSTGRES_USER: mmuser
+      POSTGRES_PASSWORD: mostest
+    healthcheck:
+      test: [ "CMD", "pg_isready", "-h", "localhost" ]
+      interval: 5s
+      timeout: 10s
+      retries: 3
+    tmpfs: /var/lib/postgresql/data
+    ports:
+      - 44446:5432
+
+  start_dependencies:
+    image: mattermost/mattermost-wait-for-dep:latest
+    depends_on:
+      - postgres
+    command: postgres:5432
+