We are using a patched version of nginx that does not erase the "ETag" http header for gzipped responses.
See: https://forum.nginx.org/read.php?2,240120,240120#msg-240120
This is what was done:
To compile nginx, the PCRE library is needed. Here is how it was installed:
$ wget ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/pcre-8.37.tar.gz
$ tar -zxvf pcre-8.37.tar.gz