ngx-voms docker image

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

docker/README.md

-This folder contains docker files for the VOMS ngx_http_voms_module.
+This folder contains a Dockerfile to run an instance of Openresty/NGINX
+compiled and linked against the ngx_voms_http_module.
+
+For more details see the [Dockerfile](./Dockerfile)
+
+The default configuration for NGINX is provided in [this conf file](
+./assets/nginx.conf). 
+
+A configuration for the `/srm` endpoint useful for the storm docker compose
+file is provided in [this conf file](./assets/srm.conf).