chore(infra): pin Docker base images by digest (DEBT-023)

All base images (debian:bookworm-slim, ubuntu:22.04, ubuntu:20.04,
rockylinux:9-minimal, centos:7, alpine:3.19, fedora:39,
kalilinux/kali-rolling, archlinux:latest, honeynet/conpot:latest)
now carry their resolved sha256 digest so 'docker pull' is
deterministic. :tag retained for human readability; @sha256 is what
Docker actually resolves. Refresh procedure documented at the top of
decnet/distros.py.

decnet/templates/https/Dockerfile

@@ -1,4 +1,4 @@
-ARG BASE_IMAGE=debian:bookworm-slim
+ARG BASE_IMAGE=debian:bookworm-slim@sha256:f9c6a2fd2ddbc23e336b6257a5245e31f996953ef06cd13a59fa0a1df2d5c252
 FROM ${BASE_IMAGE}
 
 RUN apt-get update && apt-get install -y --no-install-recommends \