Bash validation for user-defined postStart commands
MR: Pending
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## Description
This is a followup to the discussion here: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192698#note_2552691790
We want to see if we can validate the bash syntax for user-defined postStart commands in a safe way.
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## Acceptance criteria
- [ ] Bash commands are validated without the risk of a server crash through malicious commands
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issue