fix: revert to uid/gid 999

[rwaffen]

fixes #150

This might be breaking for all users who have used the newer images after #131 . We have mistakenly switched the uid/gid to 1001 and are now reverting it back to 999.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rwaffen]

@bootc pls have a look

[bootc]

This looks very odd, what's that about? On Alpine, if the OPENVOX_USER_GID is 999, the OPENVOX_GROUP should be called ping?

[bootc]

Oh this is because of #150 (comment). Tricky. I wonder whether the answer is to just have different UID/GID numbers on Ubuntu vs Alpine?