Commands are not run inside the docker image specified for a build step

Summary

Commands are not run inside the docker image specified for a build step.

Steps to reproduce

Run gitlab-runner exec docker docker_execution_test in the console.

The contents of the .gitlab-ci.yml file is:

docker_execution_test:
  image: debian:9

  script:
    - pwd
    - la

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behaviour, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug has been fixed in a more recent version)

What is the current bug behavior?

Instead of executing the commands inside the container based on the image specified, they are run in the gitlab-runner docker container:

Runtime platform                                    arch=amd64 os=darwin pid=65390 revision=3afdaba6 version=11.5.0
WARNING: You most probably have uncommitted changes.
WARNING: These changes will not be tested.
Running with gitlab-runner 11.5.0 (3afdaba6)
Using Docker executor with image debian:9 ...
Pulling docker image debian:9 ...
Using docker image sha256:4879790bd60d439cfe39c063660eef7af525d5f6f1cbb701a14c7cfc11cbfcf7 for debian:9 ...
Running on runner--project-0-concurrent-0 via theia.local...
Cloning repository...
Cloning into '/builds/project-0'...
done.
Checking out bb973ec4 as master...
Skipping Git submodules setup
$ pwd
/builds/project-0
$ ls
README.md
Job succeeded

What is the expected correct behavior?

db@theia:~/git/docker_test (master*)$ docker run -it --rm debian:9 ls
bin   dev  home  lib64	mnt  proc  run	 srv  tmp  var
boot  etc  lib	 media	opt  root  sbin  sys  usr
db@theia:~/git/docker_test (master*)$ docker run -it --rm debian:9 pwd
/

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:
Current User:	git
Using RVM:	no
Ruby Version:	2.4.5p335
Gem Version:	2.7.6
Bundler Version:1.16.6
Rake Version:	12.3.1
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.1
Go Version:	unknown
GitLab information
Version:	11.5.1
Revision:	c90ae59
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://gitlab.easybell.de
HTTP Clone URL:	https://gitlab.easybell.de/some-group/some-project.git
SSH Clone URL:	git@gitlab.easybell.de:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:
GitLab Shell
Version:	8.4.1
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Checking GitLab Shell ...
GitLab Shell version >= 8.4.1 ? ... OK (8.4.1)
hooks directories in repos are links: ...
2/1 ... ok
11/2 ... ok
15/3 ... ok
18/6 ... ok
18/7 ... ok
15/9 ... ok
15/10 ... ok
15/11 ... ok
15/12 ... ok
15/13 ... ok
15/14 ... ok
14/15 ... ok
15/16 ... ok
20/17 ... ok
18/18 ... ok
18/19 ... ok
20/20 ... ok
15/21 ... ok
15/22 ... ok
23/23 ... ok
15/24 ... ok
15/25 ... ok
14/27 ... ok
2/28 ... ok
23/29 ... ok
15/30 ... ok
15/31 ... ok
20/32 ... ok
20/33 ... ok
23/34 ... ok
10/36 ... ok
10/37 ... ok
15/38 ... ok
14/39 ... ok
20/40 ... ok
23/41 ... ok
10/42 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Reply by email is disabled in config/gitlab.yml
Checking LDAP ...
LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
2/1 ... yes
11/2 ... yes
15/3 ... yes
18/6 ... yes
18/7 ... yes
15/9 ... yes
15/10 ... yes
15/11 ... yes
15/12 ... yes
15/13 ... yes
15/14 ... yes
14/15 ... yes
15/16 ... yes
20/17 ... yes
18/18 ... yes
18/19 ... yes
20/20 ... yes
15/21 ... yes
15/22 ... yes
23/23 ... yes
15/24 ... yes
15/25 ... yes
14/27 ... yes
2/28 ... yes
23/29 ... yes
15/30 ... yes
15/31 ... yes
20/32 ... yes
20/33 ... yes
23/34 ... yes
10/36 ... yes
10/37 ... yes
15/38 ... yes
14/39 ... yes
20/40 ... yes
23/41 ... yes
10/42 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.4.5)
Git version >= 2.9.5 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 22
Checking GitLab ... Finished

### Possible fixes

From looking at the runners source code, this should be where the cmd is stored for execution within the container: https://gitlab.com/gitlab-org/gitlab-runner/blob/master/executors/docker/executor_docker.go#L860