Unable to build in parallel using the Googles Gcloud Bazel builder
If you want to run multiple bazel commands in parallel, they each need their own --output_base flag. That will make each build independent.
If you want them to share intermediate outputs within the same build machine, a shared --disk_cache is the simplest approach. You could also set up a full remote cache which would cache outputs across build machines. If you want parallel builds to deduplicate common actions fully, you'll need to set up remote execution.