Fix Key Generation Docs - resolves matrix-org#1759

This fixes the issue found in matrix-org#1759 which broke due to go changes. The new command allows you to generate keys with docker, and drop them in the current working directory.

build/docker/README.md

@@ -37,12 +37,17 @@ runtime config should come from. The mounted folder must contain:
 To generate keys:
 
 ```
-go run github.com/matrix-org/dendrite/cmd/generate-keys \
-  --private-key=matrix_key.pem \
-  --tls-cert=server.crt \
-  --tls-key=server.key
+docker run --rm --entrypoint="" \
+  -v $(pwd):/mnt \
+  matrixdotorg/dendrite-monolith:latest \
+  /usr/bin/generate-keys \
+  -private-key /mnt/matrix_key.pem \
+  -tls-cert /mnt/server.crt \
+  -tls-key /mnt/server.key
 ```
 
+The key files will now exist in your current working directory, and can be mounted into place.
+
 ## Starting Dendrite as a monolith deployment
 
 Create your config based on the `dendrite.yaml` configuration file in the `docker/config`