ci: pass USE_BAZEL_VERSION to docker (#6739)
ci: pass USE_BAZEL_VERSION to docker (#6739)

* ci: pass USE_BAZEL_VERSION to docker

This lets us easily run our existing bazel builds with different
versions of bazel. Examples

```
$ ci/cloudbuild/build.sh -t asan-pr --docker
$ USE_BAZEL_VERSION=3.5.0 ci/cloudbuild/build.sh -t asan-pr --docker

$ USE_BAZEL_VERSION=latest ci/cloudbuild/build.sh -t asan-pr --docker

$ USE_BAZEL_VERSION=last_green ci/cloudbuild/build.sh -t asan-pr --docker
```

* documented which env vars are inherited by docker builds
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp