upgrade melpazoid runtime checks (Dockerfile) to run on emacs 29.1

Makefile

@@ -1,6 +1,7 @@
 IMAGE_NAME = melpazoid
 DOCKER ?= docker
 DOCKER_OPTIONS = --cap-drop all --security-opt=no-new-privileges --pids-limit=5
+DOCKER_OUTPUT = --quiet  # e.g. '--progress=plain' xor '--quiet'
 
 .PHONY: run
 run:
@@ -17,7 +18,7 @@ term: image
 
 .PHONY: image
 image:
-	@$(DOCKER) build --build-arg PACKAGE_MAIN --quiet \
+	@$(DOCKER) build --build-arg PACKAGE_MAIN ${DOCKER_OUTPUT} \
 		--tag ${IMAGE_NAME} -f docker/Dockerfile .
 
 .PHONY: test-melpazoid

docker/Dockerfile

@@ -1,13 +1,22 @@
-FROM ubuntu:20.04
+# With big thanks to @Silex and @purcell, this Dockerfile is based on:
+# <https://github.com/Silex/docker-emacs/blob/master/29.1/debian/Dockerfile>
 
-# required to silently install libxml2-dev
-ENV DEBIAN_FRONTEND=noninteractive
+FROM nixos/nix:2.19.1 AS nixos
+
+ADD https://api.github.com/repos/purcell/nix-emacs-ci/git/refs/heads/master /tmp/cache
+RUN nix-env -iA cachix -f https://cachix.org/api/v1/install \
+    && cachix use emacs-ci \
+    && nix-env -iA emacs-29-1 -f https://github.com/purcell/nix-emacs-ci/archive/master.tar.gz \
+    && nix --extra-experimental-features nix-command copy --no-require-sigs --to /nix-emacs "$(command -v emacs)" \
+    && ln --relative --symbolic /nix-emacs/nix/store/*emacs* /nix-emacs/nix/store/emacs
+
+FROM debian:trixie-20231120-slim
+
+COPY --from=nixos /nix-emacs/nix/store /nix/store
+ENV PATH="/nix/store/emacs/bin:$PATH"
 
 RUN apt-get update \
-        && apt-get -y --no-install-recommends install software-properties-common \
-        && apt-add-repository -y ppa:kelleyk/emacs \
-        && apt-get -y --no-install-recommends install emacs28 \
-        && apt-get -y purge software-properties-common \
+        && apt-get -y install curl gnupg openssh-client wget \
         && apt-get -y autoremove \
         && rm -rf /var/lib/apt/lists/* /root/.cache/*