Tweak Dockerfile.cloud-ess

Some Docker builds are apparently complaining about the `COPY` syntax in
this file, so try and fix it as Docker suggests.

distribution/docker/src/docker/Dockerfile.cloud-ess

@@ -2,7 +2,7 @@ FROM ${base_image} AS builder
 
 USER root
 
-COPY plugins/*.zip /opt/plugins/archive
+COPY plugins/*.zip /opt/plugins/archive/
 
 RUN chown root.root /opt/plugins/archive/*
 RUN chmod 0444 /opt/plugins/archive/*