chore: add integration test running in docker container (#2613)

In this PR:
- In the integration test, run `generate_repo.py` and
`generate_pr_description.py` in docker container.
- Remove test image in cloud build.

Context: 
In this
[log](https://github.com/googleapis/google-cloud-java/actions/runs/8445620169/job/23133097820),
the nightly generation job only took 3 mins. After investigation, we
found out that removing `main` method in `generate_repo.py` (#2598)
caused the CLI is not running despite that the job returns successfully.

This non-explicit failure demonstrated that there's difference between
calling CLI (`generate` method in `generate_repo.py`) and its
implementation (`generate_from_yaml` method in `generate_repo.py`).

We decided to change the integration test to run the CLI in docker
container, rather than testing `generate_from_yaml` because it is the
containerized CLI will be used in downstream libraries.

Author: JoeWang1127

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

@@ -48,8 +48,6 @@ jobs:
       shell: bash
       run: |
         set -x
-        git config --global user.email "[email protected]"
-        git config --global user.name "Github Workflow"
         python -m unittest library_generation/test/integration_tests.py
   unit_tests:
     strategy: