gitlab-runner-build: not found

I have a very simple project in PHP. Mostly static content; little PHP code and no DB connections. I'd still like to have a deploy button in Gitlab.com. The production code is hosted in a plain ubuntu VPS.

I have this CI configuration to make a pull from the server when a deploy is made to the branch production (I tried with several images, including ubuntu, debian and busybox):

stages:
  - deploy

deploy1:
  stage: deploy
  image: ubuntu
  only: 
    - production
  before_script:
    - apt-get update
    - apt-get install --no-install-recommends --assume-yes --quiet openssh-client
  script:
    - echo "Passing..."
  after_script:
    - rm /tmp/ssh_key.pem

(I chopped the script part just to make sure there wasn't anything wrong there) When I manually run the job I get this:

Running with gitlab-runner 13.5.0 (ece86343)
  on docker-auto-scale 72989761
Preparing the "docker+machine" executor
Using Docker executor with image ubuntu ...
Pulling docker image ubuntu ...
Using docker image sha256:d70eaf7277eada08fca944de400e7e4dd97b1262c06ed2b1011500caa4decaf1 for ubuntu with digest ubuntu@sha256:fff16eea1a8ae92867721d90c59a75652ea66d29c05294e6e2f898704bdb8cf1 ...
Preparing environment
00:01
/entrypoint: line 21: exec: gitlab-runner-build: not found
ERROR: Job failed (system failure): prepare environment: exit code 127. Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information

A few days later, I tried again with the same setup and got the same error with a different version of the runner:

Running with gitlab-runner 13.6.0-rc1 (d83ac56c)
  on docker-auto-scale fa6cab46
Preparing the "docker+machine" executor
Using Docker executor with image debian:latest ...
Pulling docker image debian:latest ...
Using docker image sha256:1510e850178318cd2b654439b56266e7b6cbff36f95f343f662c708cd51d0610 for debian:latest with digest debian@sha256:8414aa82208bc4c2761dc149df67e25c6b8a9380e5d8c4e7b5c84ca2d04bb244 ...
Preparing environment
00:03
/entrypoint: line 21: exec: gitlab-runner-build: not found
ERROR: Job failed (system failure): prepare environment: exit code 127. Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information

I've been fighting hard with it but couldn't get it to work. Any help? Thanks!