add unit tests for build.go

**What would you like to be added**:
I would like to add tests for `pkg/cluster/build.go`

**Why is this needed**:
adding tests would increase security of the code

Read pkg/cluster/build.go first to understand the build behavior and identify paths that need coverage. Add unit tests for the relevant behavior, then run the Go tests for the package and confirm the new tests pass.

Written by the indexing model from the issue text.