fix: update ENTRYPOINT path to match actual binary location in Docker image

[Starosdev]

Bug

The ENTRYPOINT in Dockerfile points to /opt/radarr/Radarr, but the actual binary is located at /opt/radarr/Radarr/Radarr after the CI build process extracts the archive.

Root cause

In the docker CI job, the archive is extracted like this:

```bash
mkdir -p radarr
tar xzf _archives/Radarr.${BUILDNAME}.${{ matrix.archive_rid }}.tar.gz -C radarr/
```

The archive contains a Radarr/ subdirectory (the build output from `_artifacts//net8.0` has a `Radarr/` directory at its root). After extraction the layout is:

```
radarr/
Radarr/ <- directory, not the binary
Radarr <- the actual binary
*.dll
...
```

COPY --chmod=755 radarr /opt/radarr then copies this as /opt/radarr/Radarr/ (a directory), so the ENTRYPOINT path /opt/radarr/Radarr resolves to a directory and the container fails to start with:

```
exec: "/opt/radarr/Radarr": is a directory
```

Fix

Update the ENTRYPOINT in Dockerfile to the correct binary path:

```dockerfile
ENTRYPOINT ["/opt/radarr/Radarr/Radarr", "-nobrowser", "-data=/config"]
```

Alternatively, fix the CI extraction step to flatten the directory so the binary lands at /opt/radarr/Radarr as the current ENTRYPOINT expects.