ERROR: Job failed: exit code 6

Hi,

I don't how to fix my code since I don't know what ERROR: Job failed: exit code 6 is. I tried to Google it, no results. Tried to lookup in closed issues, still no results. Do you have a table with CI error codes somewhere?

Anyways, here's the runner code:

stages:
  - build
  - deploy

build:
  stage: build
  image: docker:git
  services:
    - docker:dind
  script:
    - apk add --update git docker
    - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $DOCKER_REGISTRY
    - docker build -t $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:master -t $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:latest -t $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:amd64 .
    - docker push $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:master
    - docker push $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:latest
    - docker push $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:amd64

deploy: 
  stage: deploy
  image: alpine:latest
  services:
    - docker:dind
  before_script:
    - apk add --update openssh-client sshpass
  script:
    - sshpass -e ssh root@$DEPLOYMENT_SERVER_IP -tt "docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $DOCKER_REGISTRY; docker pull $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:latest; docker run $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:latest"

And here's the GitLab CI log

Running with gitlab-runner 11.5.0-rc1 (e900028d)
  on docker-auto-scale ed2dce3a
Using Docker executor with image alpine:latest ...
Starting service docker:dind ...
Pulling docker image docker:dind ...
Using docker image sha256:edbe3f3ad406799b528fe6633c5553725860566b638cdc252e0520010436869f for docker:dind ...
Waiting for services to be up and running...
Pulling docker image alpine:latest ...
Using docker image sha256:196d12cf6ab19273823e700516e98eb1910b03b17840f9d5509f03858484d321 for alpine:latest ...
Running on runner-ed2dce3a-project-9278707-concurrent-0 via runner-ed2dce3a-srm-1542134875-28391ad7...
Cloning repository...
Cloning into '/builds/group/repository'...
Checking out 41a2b094 as dev...
Skipping Git submodules setup
$ apk add --update openssh-client sshpass
fetch http://dl-cdn.alpinelinux.org/alpine/v3.8/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.8/community/x86_64/APKINDEX.tar.gz
(1/3) Installing openssh-keygen (7.7_p1-r3)
(2/3) Installing openssh-client (7.7_p1-r3)
(3/3) Installing sshpass (1.06-r0)
Executing busybox-1.28.4-r1.trigger
OK: 7 MiB in 16 packages
$ sshpass -e ssh root@$DEPLOYMENT_SERVER_IP -tt "docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $DOCKER_REGISTRY; docker pull $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:latest; docker run $DOCKER_REGISTRY/$DOCKER_REGISTRY_REPO:latest"
ERROR: Job failed: exit code 6

I've double-checked the environment variables in the repository settings and they are valid. I have an active Ubuntu machine on DigitalOcean that is up and running. So, what does ERROR: Job failed: exit code 6 means?

Regards