Fix buildx problem

Error: buildx failed with: ERROR: failed to build: failed to solve: variable expansion is not supported for --from, define a new stage with FROM using ARG from global scope as a workaround

Author: physikerwelt

apache_assets/Dockerfile

@@ -1,5 +1,6 @@
 ARG WB_IMAGE=ghcr.io/mardi4nfdi/wikibase-production:latest
 ARG APACHE_IMAGE=ghcr.io/mardi4nfdi/apache:latest
 
+FROM ${WB_IMAGE} AS wb_stage
 FROM ${APACHE_IMAGE}
-COPY --from=${WB_IMAGE} /var/www/html/w /var/www/html/w
+COPY --from=w /var/www/html/w /var/www/html/w