fix manifest start_url when using SUBFOLDER

[Briffou]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Fix the start_url parameter for the PWA when SUBFOLDER is used.

Benefits of this PR and context:

When using the SUBFOLDER parameter of the docker image, the resulting PWA was using the wrong url when starting.

How Has This Been Tested?

docker run --rm -it \
  --shm-size=1gb \
  -p 3001:3001 \
  --device /dev/dri \
  -e SUBFOLDER=/test/ \
  -e PIXELFLUX_WAYLAND=true \
  -e AUTO_GPU=true \
  test_pwa_subfolder

Then install the PWA (tested on chrome 131.0.6778.69, Fedora 44) , and launch it. I checked the resulting manifest.json to look at the start_url: /test/ parameter.

I also tested without -e SUBFOLDER=/test/.

Source / References:

https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Manifest/Reference/start_url

(by the way, didn't find the changelog in the repo)

[github-actions]

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[thelamer]

Yeah baseimages are the wild west kind of there is no official versioning or changelogs:
https://github.com/linuxserver/docker-baseimage-selkies/commits/master/.

Might need to re-think that for Selkies, though clearly the goal is to never make breaking changes.
I'll get this into all the branches tomorrow.