The previous behaviour relied on managesieve copying over the
.dovecot.sieve file into the user's directory. I found this to be
particularly fragile. For instance, re-deploying roundcube without
dovecot could overwrite the .dovecot.sieve symlink and break
managesieve.

A better approach is to use the global sieve configuration that
roundcube provides and not mess with dovecot's files directly.