Allow ability to override image entrypoint in runner configuration for docker executor

Description

When pipelines are using an image that has entrypoint which doesn't start a shell when bash/sh is passed as an argument, users need to make changes to each pipeline to override the entrypoint of the image.

This issue is to ask for a configuration option in [runners.docker] section to apply overriding entrypoint for image used for the jobs.

Proposal

A new configuration option to set the image entrypoint. If entrypoint is defined in the .gitlab-ci.yml, that takes precedence.

[runners.docker]
  entrypoint = ""

Links to related issues and merge requests / references

Support ticket - internal only

Implementation steps

• Placeholder