Help with custom beets convert plugin script

[Etsum]

Description

I'm trying to use a custom python script to convert from flac to mp3, and in the process add replay gain tags and soundcheck tags so that the files are compatible with an iPod classic.

Beets convert plugin is being called on import but is not using the command specified, or the command is failing somewhere that doesn't show in the logs.

I'm running beets as a docker container using the lsio image and happy to share the compose if needed.

The relevant section of my config.yaml file is below.
The script is available at https://gist.github.com/Etsum/768948a13fc8fc2591369bfb85066115

Any help is appreciated.

Validating the script and Expected/Real Output

Running this command the script works

docker exec -u abc -it beets bash
python3 /scripts/flac-to-mp3-soundcheck.py --V0 /data/sample.flac /data/output.mp3
eyeD3 output.mp3
Time: 03:16	MPEG1, Layer III	[ ~236 kb/s @ 44100 Hz - Joint stereo ]
-------------------------------------------------------------------------------
ID3 v2.4:
title: 02. Oxford Comma
artist: Vampire Weekend
album: Vampire Weekend
recording date: 2008
track: 2		
Comment: [Description: ID3v1 Comment] [Lang: eng]
Visit https://vampire-weeken
Comment: [Description: ] [Lang: eng]
Visit https://vampire-weekend.bandcamp.com
Comment: [Description: iTunNORM] [Lang: eng]
 00002564 00002564 00005D7A 00005D7A 00000000 00000000 00008000 00008000 00000000 00000000
UserTextFrame: [Description: REPLAYGAIN_TRACK_GAIN]
-9.81 dB
UserTextFrame: [Description: REPLAYGAIN_TRACK_PEAK]
1.000000

What is output

Time: 03:16	MPEG1, Layer III	[ ~236 kb/s @ 44100 Hz - Joint stereo ]
---------------------------------------------------------------------------------
ID3 v2.4:
title: Oxford Comma
artist: Vampire Weekend
album: Vampire Weekend
album artist: Vampire Weekend
composer: Vampire Weekend
original release date: 2007-04
recording date: 2007-04
track: 2/13		
disc: 1/1
BPM: 0
Publisher/label: Beggars Japan
Unique File ID: [http://musicbrainz.org] : 96a6cbf8-228f-4da9-868d-9f5b13741377
Comment: [Description: ID3v1 Comment] [Lang: eng]
Visit https://vampire-weeken
Comment: [Description: ] [Lang: eng]
Visit https://vampire-weekend.bandcamp.com
Lyrics: [Description: ] [Lang: XXX]
UserTextFrame: [Description: Script]
Latn
UserTextFrame: [Description: ASIN]
B0010MTL70
UserTextFrame: [Description: BARCODE]
4943674077397
UserTextFrame: [Description: ARTISTS]
Vampire Weekend
UserTextFrame: [Description: CATALOGNUMBER]
WPCB-10048
UserTextFrame: [Description: ALBUMARTISTS]
Vampire Weekend
UserTextFrame: [Description: ARTISTS_SORT]
Vampire Weekend
UserTextFrame: [Description: MusicBrainz Album Type]
album
UserTextFrame: [Description: ALBUM_ARTISTS]
Vampire Weekend
UserTextFrame: [Description: Artist Credit]
Vampire Weekend
UserTextFrame: [Description: ARTISTS_CREDIT]
Vampire Weekend
UserTextFrame: [Description: ALBUMARTISTSORT]
Vampire Weekend
UserTextFrame: [Description: ALBUMARTISTS_SORT]
Vampire Weekend
UserTextFrame: [Description: MusicBrainz Album Status]
Official
UserTextFrame: [Description: ALBUMARTISTS_CREDIT]
Vampire Weekend
UserTextFrame: [Description: Album Artist Credit]
Vampire Weekend
UserTextFrame: [Description: MusicBrainz Album Release Country]
JP
UserTextFrame: [Description: Acoustid Id]
de5b4894-8c55-4197-adcc-cb009ec220c0
UserTextFrame: [Description: MusicBrainz Work Id]
81a15a2f-bf7b-3f32-846d-daf85fa47583
UserTextFrame: [Description: MusicBrainz Album Id]
29436531-8e43-4e5e-9d36-037e775e2883
UserTextFrame: [Description: MusicBrainz Artist Id]
af37c51c-0790-4a29-b995-456f98a6b8c9
UserTextFrame: [Description: MusicBrainz Album Artist Id]
af37c51c-0790-4a29-b995-456f98a6b8c9
UserTextFrame: [Description: MusicBrainz Release Group Id]
6719eb4c-42fb-3118-84a2-98c2fdbca79f
UserTextFrame: [Description: MusicBrainz Release Track Id]
a9dff027-db01-35c6-9ab0-c6b86ae04f9f
UserTextFrame: [Description: Acoustid Fingerprint]
...

Configuration

# Export #####
convert:                    
  auto: no                
  auto_keep: yes
  tmpdir: /tmpdir
  command: python3 /scripts/flac-to-mp3-soundcheck.py --V0 "$source" "$dest"
  no_convert: format:MP3
  dest: /data/plex/mp3-transcoded                        
  paths:
    default: $albumartist/$album%aunique{} ($year)/$title
  copy_album_art: yes        
  embed: yes                        
  never_convert_lossy_files: yes
  quiet: false              
  permissions:
    file: 666
    dir: 777`