fix: use BuildKit instead of legacy builder to build the Hermetic Build images

.cloudbuild/library_generation/cloudbuild-library-generation-push-prod.yaml

@@ -30,6 +30,8 @@ steps:
       "--file", ".cloudbuild/library_generation/library_generation.Dockerfile", "."]
     id: library-generation-build
     waitFor: ["-"]
+    env: 
+      - 'DOCKER_BUILDKIT=1'
 
 options:
   logging: CLOUD_LOGGING_ONLY

.cloudbuild/library_generation/cloudbuild-library-generation-push.yaml

@@ -30,6 +30,8 @@ steps:
       "--file", ".cloudbuild/library_generation/library_generation.Dockerfile", "."]
     id: library-generation-build
     waitFor: ["-"]
+    env: 
+      - 'DOCKER_BUILDKIT=1'
 
 options:
   logging: CLOUD_LOGGING_ONLY

hermetic_build/DEVELOPMENT.md

@@ -167,9 +167,6 @@ python hermetic_build/library_generation/cli/entry_point.py generate \
    ```
    Please note that the build only works when using the new
    [Docker BuildKit](https://docs.docker.com/build/buildkit/) (enabled through the `DOCKER_BUILDKIT` variable).
-   This is meant for local development only (in CloudTops) - GH Actions' Ubuntu-22.04 \
-   [comes with the latest Docker version](https://github.com/actions/runner-images/blob/e74605cd6d5407469cf224802f25057bafc23d70/images/ubuntu/Ubuntu2204-Readme.md?plain=1#L81-L83)
-   and is able to handle the build properly using the (updated) legacy builder.
 
 3. Set the version of gapic-generator-java
 

hermetic_build/library_generation/tests/integration_tests.py

@@ -202,6 +202,7 @@ def __build_image(cls, docker_file: str, cwd: str):
         subprocess.check_call(
             ["docker", "build", "-f", docker_file, "-t", image_tag, "."],
             cwd=cwd,
+            env=dict(os.environ, DOCKER_BUILDKIT="1")
         )
 
     @classmethod