generate.pl: set an always-accessible WORKDIR for any user

Ensure Perl containers can run even when the working `USER` is altered
by using `WORKDIR /`, as non-root users are unable to access `/root`.
This is especially noticeable in Kubernetes clusters (where container
processes are prevented from running as root.)

Fixes #72.

generate.pl

@@ -299,6 +299,6 @@ RUN {{docker_slim_run_install}} \
     && {{docker_slim_run_purge}} \
     && rm -fr ./cpanm /root/.cpanm /usr/src/perl /usr/src/{{cpanm_dist_name}}* /tmp/*
 
-WORKDIR /root
+WORKDIR /
 
 CMD ["perl{{version}}","-de0"]