nix: Add Docker image

Docker image has been requested for ages so let’s add it.

flake.nix

@@ -80,6 +80,11 @@
           };
 
           packages = {
+            selfoss-docker = pkgs.callPackage ./utils/docker.nix {
+              inherit (nixpkgs.lib) nixosSystem;
+              targetPlatform = system;
+            };
+
             selfoss = pkgs.callPackage ./utils/selfoss.nix {
               src = self.outPath;
             };

utils/docker.nix

@@ -0,0 +1,86 @@
+{
+  coreutils,
+  bash,
+  util-linux,
+  dockerTools,
+  unzip,
+  lib,
+  nixosSystem,
+  targetPlatform,
+}:
+
+# We are going to abuse the NixOS module for Apache to create configuration files.
+# Then we will extract them from the instantiated system and add them to the image.
+
+let
+  # Configuration for NixOS system.
+  systemConfiguration = { config, lib, pkgs, ... }: {
+    services.httpd = {
+      enable = true;
+
+      # TODO: make this overridable? Or hidden?
+      adminAddr = "admin@selfoss";
+
+      # TODO: is root okay?
+      user = "root";
+      group = "root";
+
+      virtualHosts."selfoss" = {
+        documentRoot = "/var/www";
+        locations."/" = {
+          index = "index.php index.html";
+        };
+      };
+
+      phpPackage = pkgs.php;
+      enablePHP = true;
+    };
+  };
+
+  # Instantiate the NixOS configuration.
+  system = nixosSystem {
+    system = targetPlatform;
+
+    modules = [
+      systemConfiguration
+    ];
+  };
+
+  apacheHttpd = system.config.services.httpd.package;
+
+in
+dockerTools.buildLayeredImage {
+  name = "selfoss";
+  tag = "latest";
+
+  contents = [
+    apacheHttpd
+
+    # TODO: remove, only for debugging
+    coreutils
+    bash
+    util-linux
+  ];
+
+  # Cargo-culted from https://sandervanderburg.blogspot.com/2020/07/on-using-nix-and-docker-as-deployment.html
+  maxLayers = 100;
+
+  extraCommands = ''
+    mkdir -p var/log/httpd var/cache/httpd var/www etc/httpd
+    ${selfoss} -d var/www
+    cp ${system.config.environment.etc."httpd/httpd.conf".source} etc/httpd/httpd.conf
+  '';
+
+  config = {
+    Cmd = [ "${apacheHttpd}/bin/apachectl" "-D" "FOREGROUND" ];
+    Expose = {
+      "80/tcp" = {};
+    };
+  };
+}
+
+# TODO: add devenv image
+# NOTE: Can be run:
+# nix build -L .#selfoss-docker
+# docker load < result
+# docker run -p 8080:80/tcp -it selfoss:latest