jobs ran with docker executor, instead of the configured shell executor

Summary

I have a runner configured to use the shell executor. No matter what I do, 30-40% of the time the jobs are run by the docker executor and of course they fail!

Steps to reproduce

Start AWS t2.micro instance, install gitlab-runner runner form the official repo and use the follwing YML:

before_script:
  - npm install

cache:
  paths:
    - .yarn-cache
    - node_modules

stages:
  - test

testAll:
  stage: test
  script:
    - npm run compile
    - npm run test
  only:
    - master

Actual behavior

~30% of the time the jobs are ran trough the docker executor in the default ruby image.

Expected behavior

I would expect that the jobs are ALWAYS ran with the shell executor as configured.

Relevant logs and/or screenshots

cat /etc/gitlab-runner/config.toml

concurrent = 1
check_interval = 0

[[runners]]
  name = "aws t2.micro runner"
  url = "https://gitlab.com/"
  token = "83******************3e"
  executor = "shell"
  shell = "bash"
  [runners.cache]

Log from a failed job:

Running with gitlab-runner 10.4.0-rc1 (fb4078b3)
  on docker-auto-scale (fa6cab46)
Using Docker executor with image ruby:2.1 ...
Using docker image sha256:ab47d3907c0761f6ec04535aad04525b16d93a370776a2dfc63903105962a268 for predefined container...
Pulling docker image ruby:2.1 ...
Using docker image ruby:2.1 ID=sha256:223d1eaa9523fa64e78f5a92b701c9c11cbc507f0ff62246dbbacdae395ffea3 for build container...
Running on runner-fa6cab46-project-4973529-concurrent-0 via runner-fa6cab46-srm-1515667863-fe784b81...
Cloning repository...
Cloning into '/builds/hypergrowthofficial/token-sale-platform'...
Checking out b8fd18d2 as master...
Skipping Git submodules setup
Checking cache for default...
FATAL: file does not exist                         
Failed to extract cache
$ npm install
/bin/bash: line 55: npm: command not found
ERROR: Job failed: exit code 1

Environment description

Machine Ubuntu 16.04 on AWS:

$ apt show  gitlab-runner
Package: gitlab-runner
Version: 10.3.0
Priority: optional
Section: admin
Maintainer: GitLab Inc. <support@gitlab.com>
Installed-Size: 49.0 MB
Provides: gitlab-ci-multi-runner
Depends: ca-certificates, git, curl, tar
Suggests: docker-engine
Conflicts: gitlab-runner-beta, gitlab-ci-multi-runner, gitlab-ci-multi-runner-beta
Replaces: gitlab-ci-multi-runner
Homepage: https://gitlab.com/gitlab-org/gitlab-runner
Download-Size: 26.6 MB
APT-Manual-Installed: yes
APT-Sources: https://packages.gitlab.com/runner/gitlab-runner/ubuntu xenial/main amd64 Packages
Description: GitLab Runner

$ uname -a
Linux ip-172-31-17-173 4.4.0-1041-aws #50-Ubuntu SMP Wed Nov 15 22:18:17 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Used GitLab Runner version

 gitlab-runner --version
Version:      10.3.0
Git revision: 5cf5e19a
Git branch:   10-3-stable
GO version:   go1.8.5
Built:        Fri, 22 Dec 2017 08:47:19 +0000
OS/Arch:      linux/amd64