docs: update Bazel public image guidance

[Haihan-Jiang]

Description

Update the Bazel builder documentation to avoid pointing users at the stale gcr.io/cloud-marketplace-containers/google/bazel image.

• Point the top-level alternatives list and Bazel README at the current Bazel public image, gcr.io/bazel-public/bazel.
• Call out that the Bazel public image is not a direct drop-in replacement for gcr.io/cloud-builders/bazel in Cloud Build.
• Add a Cloud Build example that sets entrypoint: bazel and a writable --output_user_root for the official image's non-root ubuntu user.
• Note that cross-step outputs/cache should use /workspace or an explicitly writable Cloud Build volume.

Fixes #927

Tests

• git diff --check
• Verified the current Bazel install docs reference gcr.io/bazel-public/bazel for Docker container usage.