Webgl build has no audio

When building the Webgl build the resulting artifact has no audio. It seems like it fails to transcode my audio assets because ffmpeg is missing in the docker container:

Updating Assets/SFX/game_over.mp3 - GUID: 755b3ad113cc6734f86e1be0eb816259...
sh: 1: ffmpeg: not found
Errors during import of AudioClip Assets/SFX/game_over.mp3:
ffmpeg -i "Temp/fsbankcache/tmpinput_Ebr0f9.WAV" -f mp4 -codec aac -b:a 192000 -strict experimental -y "Temp/importedAudioTempPrimary"
FSBTool ERROR: Error converting Temp/fsbankcache/tmpinput_Ebr0f9.WAV: make sure you have avconv or ffmpeg installed
FSBTool ERROR: Failed encoding audio clip 'Assets/SFX/game_over.mp3' to AAC. Possibly the file is too short. Try to append silence such that the length becomes greater than 256 samples.

As workaround I added the following line to my before_script:

before_script:
  - apt-get update && apt-get install -y ffmpeg

Which works, but I'm sure there is a better way to fix this.

I'm using the image gableroux/unity3d:2018.3.2f1-webgl and only build the webgl target by the way.