Hi Tobias, thank you for taking the time to create your answer!
If i may, i would like to go over your reply, so that you might be able to correct any mistakes in my perception 
The docker-compose.yml you quoted is the Docker Compose .yml file example from Install the Server | SonarQube Docs with the following one line changed, right?
-
- /path/to/your/plugins:/opt/sonarqube/extensions instead of -
- sonarqube_extensions:/opt/sonarqube/extensions
which makes the extensions bind-mounted instead of volume mounted. If i understand correctly, then:
-
Using the bind-mount instead of the volume would embed the directories content into the container and the volume would stay empty (probably?
) -
to get the plugin(s) into the volume i [c|w]ould
-
run the docker-compose with volumes and install plugins via marketplace (works for CE, but not for DE/EE/DCE because there plugins now have to be installed manually, if i understand correctly) so that they get stored in the mounted volume
-
create a dummy-container and in that process could inject them into the volume
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create my own Dockerfile/image based on “FROM sonarqube:$tag” where i COPY all plugin-jars to the right directories (because on first mount, any contents of the directory get copied into the volume)
-
Anything obviously wrong in that summary? 
cheers
Daniel
P.S.: I still would like to suggest that an addition to Install a Plugin | SonarQube Docs would be beneficial to everyone trying to install plugins while using docker. (And even more when not on CE, because then you have to install them manually now, if i understand it correctly)