SPARK-1390: Add Mkdocs to the docker file

Dockerfile

@@ -61,7 +61,7 @@ RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
  mono-reference-assemblies-4.0
 
 
-run pip install pybars3 paramiko
+run pip install pybars3 paramiko mkdocs
 run npm install -g npx
 
 # fix locale so we can run tmux