added build step

Author: mbernier

README.md

@@ -81,7 +81,8 @@ The simplest local development workflow is by using docker.
 > Steps
 
 1. Install Docker
-2. Run `docker-compose up` (This runs tests by default)
+2. Run `docker-compose build` (This builds the container)
+3. Run `docker-compose up` (This runs tests by default)
 
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