Add ability to set user UID/GID inside container


### Discussed in https://github.com/SuperioOne/nut_webgui/discussions/46

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<sup>Originally posted by **jwest1024** May 22, 2026</sup>
nut_webgui seems hardcoded to run as user 1000, which conflicts with an already existing user. I see no env or cmdline option to specify "run as uid X gid Y". I must be missing something obvious....</div>