Add buildImage shell function for building lxc-* tarballs with host keys

lxc-* hosts need NIXOS_HOST_KEYS_DIR + --impure to bake in a pre-seeded
SSH host key, otherwise sops-nix's .sops.yaml recipient never matches
and every secret permanently fails to decrypt on first boot. That
invocation is easy to forget, so wrap it as `buildImage <flake-target>`
alongside the existing Switch-nix/Test-nix helpers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/common/aliases.nix

@@ -17,6 +17,26 @@ let
       --refresh \
       --flake git+https://${vars.lanDomain}/beatzaplenty/nixos.git#$(cat /etc/flake-target)
   '';
+
+  # lxc-* hosts pre-seed their SSH host key at build time (see
+  # modules/platforms/lxc.nix) so sops-nix's .sops.yaml recipient matches on
+  # first boot -- without it, secrets permanently fail to decrypt (see that
+  # file's comment for the confirmed failure). That requires --impure plus
+  # NIXOS_HOST_KEYS_DIR pointing at the repo's host-keys/ dir, same pattern
+  # docs/auto-installer.md uses for the installer ISO. A function, not a
+  # shellAlias, since the target name has to interpolate into the middle of
+  # the flake attribute path, not just append after it. Must be run from the
+  # repo root, same as every other host-keys/ command in this repo.
+  buildImageFn = ''
+    buildImage() {
+      if [ -z "$1" ]; then
+        echo "usage: buildImage <flake-target>  (e.g. lxc-docker)" >&2
+        return 1
+      fi
+      NIXOS_HOST_KEYS_DIR="$(pwd)/host-keys" nix build --impure \
+        ".#nixosConfigurations.$1.config.system.build.tarball"
+    }
+  '';
 in
 {
   programs.bash = {
@@ -25,5 +45,6 @@ in
       "Switch-nix" = mySwitchCmd;
       "Test-nix" = myTestCmd;
     };
+    initExtra = buildImageFn;
   };
 }