Clear the puma pidfile when docker restarts

Inspired by
docker/compose#1393 (comment)

Fixes #1261

Author: jcoyne

docker-compose.yml

@@ -123,7 +123,7 @@ services:
 
   web:
     extends: app
-    command: bundle exec rails server -p 3000 -b '0.0.0.0'
+    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails server -p 3000 -b '0.0.0.0'"
     depends_on:
       - db
       - solr