fix(ci): add non-root USER to cypress snapshot Dockerfile (#41823)

## Description

Switch to the built-in `node` user (provided by the `cypress/factory`
base image) before the ENTRYPOINT to avoid running the container process
as root.

The `cypress/factory` image already sets `chmod 777 /root` so the
Cypress binary cache at `/root/.cache/Cypress` remains accessible to
non-root users. This is the officially supported pattern from the
Cypress Docker images project.

Also cleans up `apt lists` after installing chromium to reduce image
layer size.

Fixes
https://linear.app/appsmith/issue/APP-15224/triage-semgrep-finding-781859531-missing-non-root-user-in

## Automation

/ok-to-test tags="@tag.Sanity"

### :mag: Cypress test results
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## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [x] No

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

## Release Notes

* **Chores**
* Optimized Cypress test container Docker image to reduce size by
streamlining installation steps and removing temporary package lists.

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<!-- This is an auto-generated comment: Cypress test results  -->
> [!TIP]
> 🟢 🟢 🟢 All cypress tests have passed! 🎉 🎉 🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/26184866538>
> Commit: 8bf6885e2644f669dbfefe1c2ad67d613fc2e934
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=26184866538&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.Sanity`
> Spec:
> <hr>Wed, 20 May 2026 20:03:46 UTC
<!-- end of auto-generated comment: Cypress test results  -->