Cannot set ‘timeout’ value on Google Container Builder service

I'm setting up some container build triggers, which, by default, have a 10 minutes timeout configuration. As sometimes my builds are just a bit longer than 10 minutes, I'm trying to change the timeout value on the yaml file to no avail.

Container builder documentation says there's a timeout value with this characteristics:

timeout
string (Duration format) […] Default time is ten minutes. A duration in seconds with up to nine fractional digits, terminated by
's'. Example: "3.5s".

But I can't get it to work on the yaml definition file. Simplifying the yaml file I've tried:

steps:
- name: 'python:3-alpine'
  args: ['end']
  entrypoint: 'bin/notify.sh'
  env:
  - 'BRANCH=$BRANCH_NAME'
  - 'SHA=$COMMIT_SHA'
  id: 'notifyend'
timeout: '1200s'

And I get this error:

error loading template: yaml: unmarshal errors: line 9: cannot
unmarshal !!str 1200s into duration.Duration

When I use timeout: '1200' I get:

error loading template: yaml: unmarshal errors: line 9: cannot
unmarshal !!str 1200 into duration.Duration

And when I use, just for trying something else, timeout: duration.seconds(1200) I get:

error loading template: yaml: unmarshal errors: line 9: cannot
unmarshal !!str duratio... into duration.Duration

Any clue on how should I fix this?