Feat: PUID/PGID support via env variables in Dockerfile

[tikhonp]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Other information:

I had permission denied errors on my /music folder because other containers in my infrastructure write data to it. To resolve this issue, I introduced the PUID and PGID environment variables into the Docker image.

[Copilot]

Pull request overview

This PR adds support for configurable user and group IDs (PUID/PGID) via environment variables to resolve permission issues when mounting volumes that are written to by other containers. The implementation includes a new entrypoint script that dynamically adjusts user permissions at container runtime.

Key changes:

• Introduces an entrypoint.sh script that modifies user/group IDs at runtime based on PUID/PGID environment variables
• Updates Dockerfile to install gosu, create a default user, and set up the new entrypoint
• Documents the feature in README.md with default PUID/PGID values of 1000

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
entrypoint.sh	New script that handles runtime user/group ID modification and executes the application as the configured user
Dockerfile	Adds user creation, gosu installation, environment variables for PUID/PGID/USER_NAME, and switches to the new entrypoint
README.md	Documents the new PUID/PGID environment variables and their default values