Fix Dockerfile so we can run via bundle exec

I had originally planned to set WORKDIR in the Dockerfile to change to
the correct directory. However it turns out that Github actions has
other ideas and it is ignored.

Instead lets wrap everything in a shell script to change to the right
directory before running.

Dockerfile

@@ -5,4 +5,4 @@ COPY . /usr/src/app
 
 RUN cd /usr/src/app && bundle install
 
-ENTRYPOINT ["ruby", "/usr/src/app/app.rb"]
+ENTRYPOINT ["/usr/src/app/entrypoint.sh"]

entrypoint.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+cd /usr/src/app && bundle exec ruby app.rb