gitlab-runner shell executor: docker: command not found

Question

i'm running 2 gitlab-runner executors:

1. shell executor (tagged: shell_executor)
2. docker executor (tagged: docker_executor)

the docker executor runs docker commands just fine, but the shell executor throws:

I've already set gitlab-runner to the docker's usergroup. Logging whoami in the CI gives: gitlab-runner. What i'm trying to achive is deploy a docker container on the host's docker service. these are the settings i'm running:

Gitlab-runners'machine: Ubuntu 18.10

.gitlab-ci.yml

stages:
  - build
  - deploy

variables:
  DOCKER_DRIVER: overlay2
  DOCKER_TLS_CERTDIR: "/certs"

before_script:
  - export PATH=$PATH:/usr/bin:/etc/docker

build_staging:
  stage: build
  services:
    - docker:19.03.1-dind
  image: docker:19.03.1
  script:
    - apk add --update --no-cache py2-pip nodejs nodejs-npm yarn
    - yarn install
    - yarn build
    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
    - docker build -t camoncash .
    - export TAG=$(echo $CI_COMMIT_SHA | cut -c 1-7)
    - docker tag camoncash:latest $CI_REGISTRY/camoncash/camsite/camsite.frontend:$TAG
    - docker tag camoncash:latest $CI_REGISTRY/camoncash/camsite/camsite.frontend:$CI_COMMIT_REF_NAME
    - docker push $CI_REGISTRY/camoncash/camsite/camsite.frontend:$TAG
    - docker push $CI_REGISTRY/camoncash/camsite/camsite.frontend:$CI_COMMIT_REF_NAME
  tags:
    - docker_executor

deploy_staging:
  stage: deploy
  script:
    - export TAG=$(echo $CI_COMMIT_SHA | cut -c 1-7)
    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
    - docker pull -u $CI_REGISTRY/camoncash/camsite/camsite.frontend:$CI_COMMIT_REF_NAME
    - docker stop camoncash || true && docker rm camoncash || true
    - docker run --name=camoncash -p 3000:3000 -itd $CI_REGISTRY/camoncash/camsite/camsite.frontend:$CI_COMMIT_REF_NAME
  tags:
    - shell_executor

/etc/gitlab-runner/config.toml

[[runners]]
  name = "DockerRunner"
  url = "https://gitlab.com/"
  token = "TOKEN"
  executor = "docker"
  [runners.custom_build_dir]
  [runners.docker]
    tls_verify = false
    image = "docker:latest"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/certs/client", "/cache"]
    shm_size = 0
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]

[[runners]]
  name = "ShellRunner"
  url = "https://gitlab.com/"
  token = "TOKEN"
  executor = "shell"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]

me atm (╯°□°)╯︵ ┻━┻

---

UPDATE: I've worked my way around the problem for now by simply installing a gitlab-runner on the host by following here: - https://docs.gitlab.com/runner/install/linux-repository.html and then registering a shell executor to run bash commands directly on the host. Best case scenario would be to have 2 different machines running 1 specific executor. I've also discovered that running 2 different executors (eg: docker and shell) that share the same config.toml creates conflicts.