Simplify compose file to only include necessary parts for testing

roles/keycloak/files/providers/keycloak-mailpass-rest/docker-compose.yml

 version: "3.8"
 services:
-  postgres:
-    container_name: postgres
-    image: postgres:latest
-    volumes:
-      - postgres_data:/var/lib/postgresql/data
-    environment:
-      POSTGRES_DB: keycloak
-      POSTGRES_USER: keycloak
-      POSTGRES_PASSWORD: keycloak
   keycloak:
     container_name: keycloak
     build: ./docker
     environment:
       KEYCLOAK_USER: admin
       KEYCLOAK_PASSWORD: admin
-      JAVA_OPTS: -Dkeycloak.profile=preview
-      DB_ADDR: postgres
-      DB_DATABASE: keycloak
-      DB_USER: keycloak
-      DB_PASSWORD: keycloak
-    depends_on:
-      - postgres
+      KEYCLOAK_DEFAULT_THEME: archlinux
+      KEYCLOAK_WELCOME_THEME: archlinux
     ports:
       - 8080:8080
-volumes:
-  postgres_data:
-    driver: local
+    volumes:
+      - ./docker/archlinux:/opt/jboss/keycloak/themes/archlinux