TL;DR: EL6 contains old libxml2/libxslt which can cause problems when using deface on a template that contains comment like <% #this %>

Longer version:
After many hours spent on debugging I realized that the issue is that when action view compiles the template, some lines are concatenated together just split by ";". This becomes problem when ERB tag contained commentary symbol #. The reason is that when deface override is applied it first converts the template to tree using Nokogiri (see Parser.convert). This returns different results on different versions of underlying library. I didn't try to compile different versions but with libxml2-2.7.6-20.el6.x86_64 and libxslt-1.1.26-2.el6_3.1.x86_64 it does not wrap the silent erb tag with "\n" so as a result, this tag ends up on the same line as previous one. The same nokogiri but on newer lib versions (libxml2-2.9.1-5.el7_1.2.x86_64, libxslt-1.1.28-5.el7.x86_64) wrapped it.

I know that right solution would be to update libraries in EL6 or ship it ourselves but this seems much more complicated process than just removing the comment line which does not have a big value anyway. It seems to be only comment in our templates.

If we agree on this as a solution it's a fix in core, so moving back to Foreman.

Applied in changeset d46b64a2e8debb5298bf58f1941ef3204e401735.

present since 1.8, should be safe to backport to 1.11 so foreman_docker can be run on el6 with latest stable, feel free to reset