fix: add gettext and Net::DNS to engine build stage

- gettext provides msgfmt needed to compile .po -> .mo locale files
  in share/GNUmakefile during engine install_share step
- Net::DNS is a required runtime dep missing from cpanm install list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile.backend

@@ -48,6 +48,7 @@ FROM ldns AS engine-build
 
 RUN apk add --no-cache \
     gcc \
+    gettext \
     make \
     musl-dev \
     perl-dev \
@@ -82,6 +83,7 @@ RUN apk add --no-cache \
     Locale::PO \
     Locale::TextDomain \
     Module::Find \
+    Net::DNS \
     Net::IP::XS
 
 COPY zonemaster-engine/ /src/engine/