Fix userns option for rootless unix socket use case in install.md

[jasyip]

Given that the container runs as nobody user (with uid 65534 by default) and host uid, which is most likely not 65534, should map to container's uid 65534

[navidys]

Hi @jasyip

I think its better to remove the USER nobody from Containerfile.
If you intrested you can update your PR and update the Containerfile instead of install.md.

Regads

[jasyip]

I think its better to remove the USER nobody from Containerfile. If you intrested you can update your PR and update the Containerfile instead of install.md.

Most prometheus Dockerfiles follow the same template of using their busybox base image and using the nobody user. If the container were to run as root instead, it would be more convenient and the original command would work. Whereas running as nobody follows the principle of least privilege and it just only needs to be able to read/write to the socket that the host user owns. Since my new command works with the nobody user, I don't see any reason to opt for root privileges as they aren't necessary in this case and carry the risk of elevated privileges.