Fix building solr

I kept hitting an error when building solr `/bin/sh: /docker-entrypoint.sh: Text file busy`, this is caused where the script is attempted to be ran before the previous `chmod` command has properly sync'd to disk. Adding in a sync fixes this trouble for me.

data/Dockerfiles/solr/Dockerfile

@@ -4,6 +4,7 @@ COPY docker-entrypoint.sh /
 
 RUN apk --no-cache add su-exec curl tzdata \
   && chmod +x /docker-entrypoint.sh \
+  && sync \
   && /docker-entrypoint.sh --bootstrap
 
 ENTRYPOINT ["/docker-entrypoint.sh"]