Improve Docker installation (#152)

docker-entrypoint-dev.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -e
+
+sh docker-entrypoint.sh
+
+# Needed to avoid "webpack-dev-server not found" error
+yarn install --check-files
+
+# Launch Rails server and webpack-dev-server using Foreman
+foreman start -p 3000