Dockerfile: invoke dumb-init on cpanm ONBUILD for signals handling

This is important for cross-compile builds (e.g. amd64 to arm64) as
cpanm/perl won't handle signals without defining them in the code, cf
Perl/docker-perl#44.  Without this,
cross-compiles will appear to stall.

Dockerfile

@@ -60,7 +60,7 @@ ONBUILD RUN if [ -r /app/aptfile ]; then \
     apt-get -y -q update \
     && apt-get -y -q --no-install-recommends install $(cat /app/aptfile); \
   fi \
-  && cpanm --notest --quiet --installdeps --with-recommends . \
+  && /usr/bin/dumb-init -- cpanm --notest --quiet --installdeps --with-recommends . \
   && apt-get purge -y -q $(perl -le'@seen{split " ", "" . do { local ($/, @ARGV) = (undef, "/app/aptfile"); <> }} = () if -r "aptfile"; print for grep { !exists $seen{$_} } qw(make gcc git openssh-client libc6-dev libssl-dev zlib1g-dev)') \
   && rm -rf /var/lib/apt/lists/* /var/cache/apt/* /root/.cpanm /tmp/*