Updated Dockerfile

Also added instructions for building and deploying Docker image, and added docker directory with custom entrypoint

docker/entrypoint

@@ -0,0 +1,18 @@
+#!/usr/bin/env sh
+# $0 is a script name,
+# $1, $2, $3 etc are passed arguments
+# $1 is our command
+CMD=$1
+
+case "$CMD" in
+  'start' )
+    echo `pwd`
+    bundle exec rails s -b 0.0.0.0
+    ;;
+
+   * )
+    # Run custom command. Thanks to this line we can still use
+    # "docker run our_image /bin/sh" and it will work
+    exec "$@"
+    ;;
+esac