feat: add Postfix/pneumatube container rig with live recipient rejection

docker/postfix/Dockerfile

@@ -0,0 +1,14 @@
+FROM debian:bookworm-slim
+
+# "No configuration" skips postfix's interactive debconf setup during install;
+# the entrypoint configures main.cf/transport itself via postconf.
+RUN echo "postfix postfix/main_mailer_type select No configuration" | debconf-set-selections \
+    && DEBIAN_FRONTEND=noninteractive apt-get update \
+    && apt-get install -y --no-install-recommends postfix \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+EXPOSE 25
+ENTRYPOINT ["/entrypoint.sh"]