Make `script:` optional since containers can already predefine a task to be run via ENTRYPOINT
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### Problem to solve
We require a `script` section now in any job, but it's possible when using containers to define an ENTRYPOINT. If your script uses a container that just does one thing, that already has an entrypoint you need to do something like the following or you will get a YAML error:
```yaml
superlinter:
image: github/super-linter:latest
script: [ "true" ]
variables: { RUN_LOCAL: "true", DEFAULT_WORKSPACE: $CI_BUILDS_DIR }
```
The `script: [ "true" ]` part doesn't actually do anything (it's not even run), but it's the only way to not get a parsing failure.
### Intended users
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### User experience goal
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### Proposal
Make `script` optional.
### Further details
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### Is this a cross-stage feature?
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### Links / references
issue