Increase proxy_buffer_size for projects with large header size

[asgorobets]

Drupal 8 sets cache tags headers and cache contexts that can exceed the limit of 8k very easily. I propose to increase it to 16k at least

[lmakarov]

This change breaks tests. I remember this same issue from a while ago - it was not possible to set this buffer above 8k without breaking tests. Not sure why.

[achekulaev]

It does not build because there are several interdependent values in the configuration and to modify one others should be modified as well. Here is proper PR https://github.com/docksal/service-vhost-proxy/pull/19

"256k should be enough for everyone" ©

[lmakarov]

Thanks @asgorobets and @achekulaev. Closing in favor of #19