Setup nginx to run on rootless mode

[davzucky]

PR Checklist:

• [x ] add a short description of what's changed to the top of the CHANGELOG.md
• [x ] add/update tests (or don't, for reasons explained below)

Describe this PR

They are more and more people running prefect server on rootless environment on Kubernetes. Some people complain about it on the slack channel .
This change force nginx to run on root less mode

[TylerWanner]

it is great to change to rootless--should we instead just use this base image? https://github.com/nginxinc/docker-nginx-unprivileged

[davzucky]

it is great to change to rootless--should we instead just use this base image? https://github.com/nginxinc/docker-nginx-unprivileged

I don't have any preference. The result would be the same. I know this is what we are using today to customize the base ui image to rootless.

I can change to use the base nginx rootless image if you prefer

[znicholasbrown]

I think that makes more sense @davzucky - would you mind changing the base image?

[davzucky]

pushed the change to use nginxinc/nginx-unprivileged:stable as a base image

[znicholasbrown]

LGTM @davzucky 👍🏻 !

[TylerWanner]

thanks @davzucky !