Add Dockerfile for application setup, update next.config.ts for standalone output, enhance documentation for FFMPEG_PATH, and implement ffmpeg location flag in download functionality

docs/configuration.md

@@ -37,14 +37,19 @@ which yt-dlp
 ### Optional Configuration
 
 #### FFMPEG_PATH
-**Optional - Reserved for future use**
+**Optional - For custom ffmpeg location**
 
-Full path to ffmpeg executable (currently not used, but reserved for future configuration).
+Full path to the ffmpeg executable. Passed to yt-dlp via `--ffmpeg-location` flag. Only needed if ffmpeg is not in your system PATH.
 
 ```bash
 FFMPEG_PATH=/usr/local/bin/ffmpeg
 ```
 
+**When to set this:**
+- ffmpeg is not installed via package manager
+- ffmpeg is in a custom location (e.g., Docker, Coolify deployments)
+- You're using a standalone ffmpeg binary
+
 ## Configuration Examples
 
 ### Example 1: yt-dlp in Custom Location