optimize basic installs

Dockerfile

@@ -1,9 +1,9 @@
 FROM debian:bookworm-slim
 LABEL maintainer="docker@ao-it.net"
 
+ARG DEBIAN_FRONTEND=noninteractive
 ## prepare requirements
 RUN ["bash", "-exo", "pipefail", "-c", "\
-    export DEBIAN_FRONTEND=noninteractive ; \
     apt update ; \ 
     apt install -y wget gnupg ; \
     wget -O - https://packages.icinga.com/icinga.key | \
@@ -16,7 +16,6 @@ RUN ["bash", "-exo", "pipefail", "-c", "\
 
 ## install icinga2 
 RUN ["bash", "-exo", "pipefail", "-c", "\
-    export DEBIAN_FRONTEND=noninteractive ; \
     apt update ; \
     apt install -y icinga2 monitoring-plugins ; \
     mkdir -p /run/icinga2 ; \