Fix hang in incremental build

Incremental builds were hanging because both build.go and docker.go were waiting on each other. To fix it, I simplified the code and no longer use multiple go routines in build.go#saveArtifacts
As mentioned on IRC, it makes sense to wait for all goroutines to finish before returning with the error.  Previously the wg.Wait() happened after the error check/return which didn't ensure that the goroutines were done.