Fix warning on dockerfile

Author: ericof

Dockerfile

@@ -5,7 +5,7 @@ LABEL maintainer="kitconcept GmbH <info@kitconcept.com>" \
       org.label-schema.description="Deploy docker stack" \
       org.label-schema.vendor="kitconcept GmbH" \
       org.opencontainers.image.source="https://github.com/kitconcept/docker-stack-deploy" \
-      org.label-schema.docker.cmd="docker run -rm -v "${PWD}":/github/workspace ghcr.io/kitconcept/docker-stack-deploy"
+      org.label-schema.docker.cmd="docker run -rm -v "$(PWD)":/github/workspace ghcr.io/kitconcept/docker-stack-deploy"
 
 RUN apk add --no-cache openssh-client findutils bash