Docker: Fix entrypoint to run as PID 1

[calvinbui]

Description

Replace su -p -c with exec su-exec when running as root, and add exec when running as non-root user.

Without exec, the shell script remains as PID 1 and nzbget runs as a child process. This means Docker's stop signal (SIGTERM) are sent to the shell, not nzbget, preventing graceful shutdown. SIGKILL is sent when the SIGTERM times out.

Using exec replaces the shell process with nzbget, making it PID 1.

There are also small improvements in the Dockerfile:

• Use COPY instead of ADD
• Fix equality comparison for sh
• Missing --no-cache argument for apk

Lib changes

N/A

Testing

I built and ran the image. When I restarted/stopped it and saw [INFO] Stopping, please wait... in the logs.

I performed the same test with ghcr.io/nzbgetcom/nzbget:v26.0 and the log message didn't appear.

[phnzb]

@calvinbui Thank you for the contribution. Tested on Ubuntu 24.04.3 LTS and Docker 28.3.3; no issues found. Merged into develop.