fix(docs): configure docs container to update (googleapis#9356)

Author: bshaffer

.kokoro/docs/common.cfg

@@ -7,44 +7,23 @@ action {
   }
 }
 
-# Download trampoline resources.
-gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
+# Path to Dockerfile relative to the SCM root
+dockerfile_path: ".kokoro/docs/docker/Dockerfile"
 
-# Use the trampoline script to run in docker.
-build_file: "google-cloud-php/.kokoro/trampoline.sh"
-
-# Configure the docker image for kokoro-trampoline.
-env_vars: {
-    key: "TRAMPOLINE_IMAGE"
-    value: "gcr.io/cloud-devrel-kokoro-resources/google-cloud-php-docs"
-}
-env_vars: {
-    key: "TRAMPOLINE_BUILD_FILE"
-    value: "github/google-cloud-php/.kokoro/docs/publish.sh"
+container_artifact {
+  destination: "gcr.io/cloud-devrel-kokoro-resources/google-cloud-php-docs:latest"
 }
 
 env_vars: {
     key: "STAGING_BUCKET"
     value: "docs-staging-v2"
 }
 
-# It will upload the docker image after successful builds.
-env_vars: {
-    key: "TRAMPOLINE_IMAGE_UPLOAD"
-    value: "true"
-}
-
-# It will always build the docker image.
-env_vars: {
-    key: "TRAMPOLINE_DOCKERFILE"
-    value: ".kokoro/docs/docker/Dockerfile"
-}
-
 before_action {
   fetch_keystore {
     keystore_resource {
       keystore_config_id: 73713
       keyname: "docuploader_service_account"
     }
   }
-}
+}