Add `grep` as a dependency when overriding an image's ENTRYPOINT

In #37297 (closed) a user has created a minimal docker image and attempted to use it as a build image. This results in the error:

sh: 3: grep: not found

And indeed https://gitlab.com/gitlab-org/gitlab-runner/-/blob/main/shells/bash.go#L331 invokes grep.

Closes #37297 (closed)