ngx-voms docker image

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

docker/assets/nginx.conf

+user build;
+worker_processes  1;
+
+env OPENSSL_ALLOW_PROXY_CERTS=1;
+env X509_VOMS_DIR=/vomsdir;
+
+error_log logs/error.log  warn;
+
+events {
+  worker_connections 1024;
+}
+
+http {
+
+    include       mime.types;
+    default_type  application/octet-stream;
+
+    log_format storm '$time_iso8601 [$request_id] $remote_addr - $remote_user "$request" <$upstream_response_time> '
+                    '$ssl_protocol/$ssl_cipher '
+                    '"$ssl_client_s_dn" '
+                    '[$voms_fqans] '
+                    '$status $body_bytes_sent "$http_referer" '
+                    '"$http_user_agent" "$http_x_forwarded_for"';
+
+    access_log logs/access.log  storm;
+
+    sendfile        on;
+    #tcp_nopush     on;
+
+    keepalive_timeout  65;
+
+    #gzip  on;
+
+    include /etc/nginx/conf.d/*.conf;
+}