Gitlab runner [build failed]

I am using gitlab runner in docker. I am unable to run my build.

Output :

Running with gitlab-ci-multi-runner 1.8.1 (a2efdd4)
Using Docker executor with image gitlab/gitlab-runner …
Using locally found image version with exactly the same ID
Pulling docker image gitlab/gitlab-runner …
Running on runner-4bd58668-project-59-concurrent-0 via 7121299397c6…
Fetching changes…
HEAD is now at 678ba0d Update file1.txt
From http://gitlabserver.com/sheik/testingrunner
678ba0d…1a58730 master -> origin/master
Checking out 1a587300 as master…
**FATAL: Command sh not found. **
FATAL: Command sh not found.
ERROR: Build failed: exit code 1

I am getting FATAL: Command sh not found error. Please help me where i am wrong.

I have exactly the same problem and did the same thing.
stages:
- custom
build_job_custom_name:
stage: custom
script:
- echo Hallo

This is all the script does, so I am very confused that this goed wrong, because it does nothing only ECHO…