Extended Docker configuration options for image entrypoint seem to not be working as documented

Summary

While following the documentation here, I was unable to successfully override the entrypoint of the image I was using in my test to /bin/sh.

Steps to reproduce

Snippet of .gitlab-ci.yml

validate_packer_file:
  stage: linting
  except:
    - tags
  image:
    name: hashicorp/packer:light
    entrypoint: ["/bin/sh"]
  script:
    - /bin/packer validate packer_relay.json

This is essentially what the documentation suggests doing if the Docker image specifies an entrypoint.

Actual behavior

When running the test, I get the following output from the Runner:

# snip
Checking out de3081e4 as ami...
Skipping Git submodules setup
/bin/sh: can't open 'sh'
/bin/sh: can't open 'sh'
ERROR: Job failed: exit code 2

After some digging, I determined this to be because the container was still being started with sh as the command, so the container was trying to execute /bin/sh sh which apparently does not work.

Expected behavior

Based on the documentation provided, I would expect my configuration to not cause an error.

Relevant logs and/or screenshots

Running with gitlab-ci-multi-runner 9.5.0 (413da38)
  on aws_docker (c00515ef)
Using Docker executor with image hashicorp/packer:light ...
Using docker image sha256:d6b9eed0071936bb6bdf482ec72618b93a30d1f033b82f0159a9251023dbb127 for predefined container...
Pulling docker image hashicorp/packer:light ...
Using docker image hashicorp/packer:light ID=sha256:36767633ab7b4958d25f19f5e4195bd4bbe57064755166c22983443aefab8c18 for build container...
Running on runner-c00515ef-project-2170336-concurrent-0 via ip-0-0-0-0...
Fetching changes...
HEAD is now at de3081e forget shared runners
Checking out de3081e4 as ami...
Skipping Git submodules setup
/bin/sh: can't open 'sh'
/bin/sh: can't open 'sh'
ERROR: Job failed: exit code 2

Environment description

I'm using Docker version 1.13.0

Used GitLab Runner version

I'm using gitlab.com and a privately hosted runner.

root@ip-0-0-0-0:~# gitlab-ci-multi-runner --version
Version:      9.5.0
Git revision: 413da38
Git branch:   9-5-stable
GO version:   go1.8.3
Built:        Tue, 22 Aug 2017 13:35:13 +0000
OS/Arch:      linux/amd64

Workaround

If I set the entrypoint to an empty string, I am able to get the desired behavior.

validate_packer_file:
  stage: linting
  except:
    - tags
  image:
    name: hashicorp/packer:light
    entrypoint: [""] # force an empty entrypoint
  script:
    - /bin/packer validate packer_relay.json

This may be the intended implementation and there is just a documentation error (or I am misinterpreting the docs).

Thank you for all the hard work! Gitlab CI is a fantastic tool.