Make "db/" into a volume inside the Docker container

AWS ECS docs [1] say that a file that exists in a directory that is
subsequently marked as a volume will be copied to the host and remounted into
the container.

This is necessary for us to do with the "db/" directory because the
"db:migrate" Rake task writes to "db/schema.rb"

[1] https://docs.aws.amazon.com/AmazonECS/latest/developerguide/bind-mounts.html

Author: AP-Hunt

Dockerfile

@@ -58,9 +58,10 @@ RUN chmod 0755 bin/*
 COPY --chown=ruby:ruby --from=build /usr/local/bundle /usr/local/bundle
 COPY --chown=ruby:ruby --from=build /app /app
 
-RUN mkdir -p "/app/tmp/" && chown ruby:ruby "/app/tmp/"
+RUN mkdir -p "/app/tmp/" && chown ruby:ruby "/app/tmp/" && chown ruby:ruby "/app/db/"
 VOLUME "/tmp/"
 VOLUME "/app/tmp/"
+VOLUME "/app/db/"
 
 EXPOSE 9292