I recently rebuilt some images that depend on the python:2.7 image. This app I'm running uses some uwsgi routing rules, and I discovered that it was broken after the rebuild. Digging into the log I found:
balrogadmin_1 | !!! no internal routing support, rebuild with pcre support !!!

If I add this to my Dockerfile, everything works again:
RUN apt-get -q update && apt-get -q --yes install libpcre3 libpcre3-dev && apt-get clean

I suspect that the apt-get purge --auto-remove from 8d03a50#diff-d6ac67f048d96870edb1787bc2b3e17cR54 ends up removing libpcre3-dev, which causes uwsgi to install without routing support, though I'm not certain of that.