docker: add keygen-httpd service to docker-compose.shell.yaml

Here is how I use it
http://frostyx.cz/posts/copr-docker-compose-without-supervisord#keygen

Author: FrostyX

docker-compose.shell.yaml

@@ -23,3 +23,6 @@ services:
 
   distgit:
     command: /bin/bash
+
+  keygen-httpd:
+    command: /bin/bash