build(docker/frontend): fix npm registry parametrization

docker/frontend/Dockerfile

@@ -30,7 +30,7 @@ WORKDIR ${PROJECT_DIR}
 ENV HOME=${PROJECT_DIR}
 
 RUN if [ ! -z "${NPM_CUSTOM_REGISTRY}" ]; then \
-printf 'registry=https://pkgs.dev.azure.com/fraport/_packaging/packages/npm/registry/\n\nalways-auth=true' > .npmrc \
+printf 'registry=${NPM_CUSTOM_REGISTRY}' > .npmrc \
 ; fi
 
 RUN make -- --dependencies-frontend IN_CONTAINER=true