build: perform validation on all base images for PRs (#1610)

tests/setuponlytests/generic-packs-prefix/docker-compose.yml

@@ -10,6 +10,12 @@ services:
       GENERIC_PACKS_PREFIX: /packs/
       GENERIC_PACKS_SUFFIX: .zip
       DEBUG: "true"
+      # the following are only used to speed up test execution
+      TYPE: CUSTOM
+      CUSTOM_SERVER: /servers/fake.jar
+      VERSION: 1.18.1
     volumes:
       - ./packs:/packs
       - ./data:/data
+      # the following are only used to speed up test execution
+      - ./verify.sh:/servers/fake.jar