Chore: update docker image & ci testing node to v18

[shamoon]

Proposed change

Even though node v20 will enter LTS in like a month we cant upgrade to that because of angular/angular-cli#25782 (which I suspect will literally be fixed in the next couple days). Not a big deal, I think, as node versions dont seem to come with huge improvements. This does resolve https://github.com/paperless-ngx/paperless-ngx/actions/runs/6123228728/job/16621724619#step:11:430

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please explain): chore

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[stumpylog]

Should the CI testing add v18 as well?

[shamoon]

It most certainly should, my bad. Fixed

[stumpylog]

I think a status got lost by GH, but looks good

[github-actions]

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