ERROR: (gcloud.beta.functions.deploy) ... message=[The caller does not have permission]

I am trying to deploy code from this repo:

https://github.com/anishkny/puppeteer-on-cloud-functions

in Google Cloud Build. My cloudbuild.yaml file contents are:

steps:
- name: 'gcr.io/cloud-builders/gcloud'
  args: ['beta', 'functions', 'deploy', 'screenshot', '--trigger-http', '--runtime', 'nodejs8', '--memory', '1024MB']

I have given the following roles to my Cloud Build Service account (****@cloudbuild.gserviceaccount.com):

• Cloud Build Service Account
• Cloud Functions Developer

Yet, in my Cloud Build log I see the following error:

starting build "1f04522c-fe60-4a25-a4a8-d70e496e2821"

FETCHSOURCE
Fetching storage object: gs://628906418368.cloudbuild-source.googleusercontent.com/94762cc396ed1bb46e8c5dbfa3fa42550140c2eb-b3cfa476-cb21-45ba-849c-c28423982a0f.tar.gz#1534532794239047
Copying gs://628906418368.cloudbuild-source.googleusercontent.com/94762cc396ed1bb46e8c5dbfa3fa42550140c2eb-b3cfa476-cb21-45ba-849c-c28423982a0f.tar.gz#1534532794239047...
/ [0 files][    0.0 B/  835.0 B]                                                
/ [1 files][  835.0 B/  835.0 B]                                                
Operation completed over 1 objects/835.0 B.                                      
tar: Substituting `.' for empty member name
BUILD
Already have image (with digest): gcr.io/cloud-builders/gcloud
ERROR: (gcloud.beta.functions.deploy) ResponseError: status=[403], code=[Forbidden], message=[The caller does not have permission]
ERROR
ERROR: build step 0 "gcr.io/cloud-builders/gcloud" failed: exit status 1

What am I missing?

According to Cloud Build documentation, for Cloud Functions you have to grant the "Project Editor" role to your service account.

But, Cloud Functions documentation states that alternatively to using the Project Editor role, you can use "the Cloud Functions Developer role [but you have to] ensure that you have granted the Service Account User role". Regarding Service Accounts, it indicates to have "the CloudFunctions.ServiceAgent role on your project" and to "have permissions for trigger sources, such as Pub/Sub or the Cloud Storage bucket triggering your function".

Due to those considerations, my understanding is that the documentation omitted to specify all the roles your service account would need and went directly to indicate to grant the Project Editor role.