Update Dockerfile to use COPY instead of ADD as recommended on https:…

…//docs.docker.com/develop/develop-images/dockerfile_best-practices/#add-or-copy (#946)

Signed-off-by: TheDen <deniskhoshaba@gmail.com>

Co-authored-by: Adam Warner <me@adamwarner.co.uk>

Dockerfile

@@ -20,7 +20,7 @@ RUN bash -ex install.sh 2>&1 && \
 
 ENTRYPOINT [ "/s6-init" ]
 
-ADD s6/debian-root /
+COPY s6/debian-root /
 COPY s6/service /usr/local/bin/service
 
 # php config start passes special ENVs into