As you know, AzuraCast comes with pre-built liquidsoap config. In order to enable any plugin, according to its authors, you will need to manually re-build the docker image, and use it as replacement for original container.
As I am not familiar much with Docker, I’ve overgone a dirty way to enable specific plugin, called soundtouch in my `Ubuntu & Docker` Azuracast installation ( So, there might be easier way, if so, please comment it)

• Put the contents of this repo into a new folder, i.e.: /var/azuracast/my-radio-image
• In its Dockerfile , where it says opam_packages .... liquidsoap , before liquidsoap, add soundtouch in the list.
• In the end of file: build/setup/liquidsoap.sh in the end of the plugins list, add libsoundtouch-dev
• then open up a terminal and run:

cd /var/azuracast && docker build -t my_custom_station ./my-radio-image

After several minutes needed, it will complete process, and note that it shouldn’t produced error. To confirm that, run docker images and in the list you should see the latest image named my_custom_station . If not, you should correct the build error.

• then in /var/azuracast create docker-compose.override.yml and insert:

version: '2.2'
services:
 stations:
     image: "my_custom_station:latest" 

Then run the command:

docker-compose up -d

After that, added function in liquidsoap configuration, restarted liquidsoap (wait several seconds in between) and it was working.

NOTE:  On every update of official AzuraCast-Radio package, you might need to do this process again, to adopt the new azura-stations image.

References

https://hub.docker.com/r/stephanel/liquidsoap/dockerfile
https://github.com/AzuraCast/AzuraCast/issues/1943
https://github.com/AzuraCast/AzuraCast/issues/3530
https://github.com/AzuraCast/AzuraCast/issues/3540