Amend the text about the starting docker image

docker/ngx-voms-build/README.md

 # Docker image with the build environment for the NGINX VOMS Module
 
 This folder contains the Dockerfile for the build environment of the NGINX VOMS
-module, as an extension of the `storm2/build` image.
+module, as an extension of the `storm2/base` image.
 
 The build environment is based on the NGINX distribution provided
 by [OpenResty](https://www.openresty.org/).