docker: extract the vichan directory and make it optionally exposable

docker/nginx/Dockerfile

@@ -2,9 +2,7 @@ FROM nginx:1.25.3-alpine
 
 COPY . /code
 RUN adduser --system www-data \
-  && adduser www-data www-data \
-  && /code/docker/common-setup.sh
-
+  && adduser www-data www-data
 
 CMD [ "nginx", "-g", "daemon off;" ]
 EXPOSE 80