Update docker readme
These are Docker images for Dendrite!
+They can be found on Docker Hub:
+
+- [matrixdotorg/dendrite-monolith](https://hub.docker.com/repository/docker/matrixdotorg/dendrite-monolith) for monolith deployments
+- [matrixdotorg/dendrite-polylith](https://hub.docker.com/repository/docker/matrixdotorg/dendrite-polylith) for polylith deployments
+
## Dockerfiles
The `Dockerfile` builds the base image which contains all of the Dendrite
