try replacing config tags with environment variables

postfix/Dockerfile

@@ -1,13 +1,16 @@
 FROM debian:bookworm-slim
 
+COPY entrypoint.sh /entrypoint.sh
+
 RUN apt-get update && \
     apt-get -y upgrade && \
     apt-get -y --purge autoremove && \
     apt-get -y install postfix postfix-pgsql postgrey && \
-    apt-get -y clean
+    apt-get -y clean && \
+    chmod a+x /entrypoint.sh
 
 EXPOSE 25/tcp
 EXPOSE 465/tcp
 EXPOSE 587/tcp
 
-CMD ["/usr/sbin/postfix", "start-fg"]
+CMD ["/entrypoint.sh"]

postfix/entrypoint.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+for cf in /etc/postfix/*.cf; do
+  sed -iE -e 's/%DB_PASSWORD%/${DB_PASSWORD}/g' \
+          -e 's/%DB_HOST%/${DB_HOST}/g' \
+          -e 's/%DB_USER%/${DB_USER}/g' \
+          -e 's/%DB_DATABASE%/${DB_DATABASE}/g' ${cf};
+done
+
+/usr/sbin/postfix start-fg