chore(ci): Permission-based ephemeral Docker build

[khvn26]

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Changes

This PR makes the Docker build image assume ephemeral build based on Github actor permissions. Fixes e.g. @dabeeeenster's PRs being regarded as external contributions.

How did you test this code?

This is a CI change.

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3 Ignored Deployments

| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **docs** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/docs/3kPcc7Au7ASq3idvY72jevqYNPR9)) | [Visit Preview](https://docs-git-chore-permission-based-ephemeral-dock-8633b3-flagsmith.vercel.app) | | Jun 25, 2024 3:57pm | | **flagsmith-frontend-preview** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-preview/FL821t3Qh5nYonbdsxGChuLK2A6K)) | [Visit Preview](https://flagsmith-frontend-preview-git-chore-permissio-ca9339-flagsmith.vercel.app) | | Jun 25, 2024 3:57pm | | **flagsmith-frontend-staging** | ⬜️ Ignored ([Inspect](https://vercel.com/flagsmith/flagsmith-frontend-staging/4ENuuysxX7A9xGS8kX6LXmhq7YAs)) | [Visit Preview](https://flagsmith-frontend-staging-git-chore-permissio-599fa1-flagsmith.vercel.app) | | Jun 25, 2024 3:57pm |

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.57%. Comparing base (b9a6f92) to head (41fbc95). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4232 +/- ## ======================================= Coverage 96.57% 96.57% ======================================= Files 1190 1190 Lines 38754 38754 ======================================= Hits 37426 37426 Misses 1328 1328 ```

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