Support having Rails project in a subdirectory

By setting the `RELATIVE_RAILS_ROOT` environment variable.

Author: michaelbaudino

entrypoints/rails-entrypoint

@@ -2,6 +2,6 @@
 
 # Remove rails server PID file, that is sometimes not properly removed at shutdown
 # as instructed in https://docs.docker.com/compose/rails/
-rm -f /app/tmp/pids/server.pid
+rm -f /app/${RELATIVE_RAILS_ROOT}/tmp/pids/server.pid
 
 exec "$@"