Quote su commands in docker docs (#44)

Quote su commands in docker docs

source/installation/with/docker.html.md

@@ -38,9 +38,9 @@ docker-compose pull
 # Launch the database container
 docker-compose up -d postgres
 # Create the database role (it will ask you for the password)
-docker-compose run --rm postgres su postgres -c createuser plume -dP
+docker-compose run --rm postgres su postgres -c 'createuser plume -dP'
 # Setup the database (create it and run migrations)
-docker-compose run --rm postgres su postgres -c createdb plume -O plume
+docker-compose run --rm postgres su postgres -c 'createdb plume -O plume'
 docker-compose run --rm plume plm database setup
 
 # Setup your instance