Installing plugins when using Docker

How do I install plugins when using prebuilt Docker images? Specifically, I'd like to install loomio_tags for the Borderland Talk Loomio. I've got my own forks of Loomio and loomio-deploy set up and my own Docker hooked to my Loomio repo. I've tried creating a plugins.yml pointing to loomio/loomio_tags but when building the docker image I get a non-zero code 11. What's going on here?