support running with custom non-root user

[coffee-cup]

This PR adds support for running the start command as a non-root user. The user is frist created with useradd.

This is a very simply implementation of custom user support and if you have a another use case that requires additonal user configuration, please leave a comment or create an issue to open up a discussion.

Closes #449, #990

Configuration

user is added as a field to the start phase and can be configured with a nixpacks.toml file like this

# ...

[start]
user = "myuser"

or via the command line by passing custom config. For example

$ nixpacks build examples/custom-user --name user --json-plan '{ "start": { "user": "myuser2" } }'

[fattila16]

When it comes to serving static assets, nginx is used. With the introduction of the custom non-root user, would this break that? As nginx needs root access to write /var/log/nginx, /var/cache/nginx and for whatever else I'm not 100% familiar what nginx needs to run so I might be wrong

[coffee-cup]

This will not change anything by default so nothing should break when nginx is used unless a non-root user is manually specified.

[github-actions]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days

[github-actions]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days