ngx-voms docker image

See merge request storm2/ngx_http_voms_module!9

add support for ssl_client_ee_s_dn and ssl_client_ee_i_dn

Closes #14

See merge request storm2/ngx_http_voms_module!8

This commit introduces the ngx-voms docker image, and modifies the CI
configuration to build the image.

Issue 5 - Write tests to cover all VOMS attributes exposed by NGINX VOMS module

See merge request storm2/ngx_http_voms_module!5

Issue 9

See merge request storm2/ngx_http_voms_module!6

# Conflicts:
#   t/README.md