generate.pl: Configure cpanm to install over HTTPS

- pass `--from https://www.cpan.org` to cpanm by default

- Ensure ca-certificates and curl are installed in slim builds so HTTPS
  installs work

Author: zakame

generate.pl

@@ -61,7 +61,7 @@ sub die_with_sample {
 chomp $docker_slim_run_install;
 
 my $docker_slim_run_purge = <<'EOF';
-savedPackages="make netbase" \
+savedPackages="ca-certificates curl make netbase" \
     && apt-mark auto '.*' > /dev/null \
     && apt-mark manual $savedPackages \
     && apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false \
@@ -280,6 +280,8 @@ =head1 DESCRIPTION
 COPY *.patch /usr/src/perl/
 WORKDIR /usr/src/perl
 
+ENV PERL_CPANM_OPT="--from https://www.cpan.org"
+
 RUN {{docker_slim_run_install}} \
     && curl -SL {{url}} -o perl-{{version}}.tar.{{type}} \
     && echo '{{sha256}} *perl-{{version}}.tar.{{type}}' | sha256sum -c - \