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;
   };
 }

scripts/env.sh

@@ -88,60 +88,13 @@ nix_extra_opts() {
   fi
   export NIX_EXTRA_OPTS_DECIDED=1
   NIX_OPTS=()
-
-  # Retry a couple of times, 1s apart, before believing either check --
-  # belt-and-suspenders against a genuine multi-second blip (nix-cache
-  # restarting), on top of the fix below. Worst case (~11s total, host
-  # genuinely gone) is still nowhere near the 15s+ *per lookup* nix's own
-  # substituter retries would cost if this check didn't exist at all.
-  local attempt cache_up=0 builder_up=0
-  for attempt in 1 2 3; do
-    if curl --silent --fail --max-time 3 "http://${NIX_CACHE_HOST}/nix-cache-info" >/dev/null 2>&1; then
-      cache_up=1
-      break
-    fi
-    [[ "$attempt" -lt 3 ]] && sleep 1
-  done
-
-  if [[ "$cache_up" -eq 0 ]]; then
+  if ! curl --silent --fail --max-time 3 "http://${NIX_CACHE_HOST}/nix-cache-info" >/dev/null 2>&1; then
     echo "nix-cache (http://${NIX_CACHE_HOST}) is unreachable -- skipping it (substituter + remote builder) for the rest of this run." >&2
     NIX_OPTS=(--option substituters "https://cache.nixos.org/" --builders "")
-  else
-    for attempt in 1 2 3; do
-      # `exec 3<>/dev/tcp/...` just opens the fd and returns -- it does NOT
-      # read from it. Confirmed live this is load-bearing, not stylistic:
-      # the previous `cat < /dev/tcp/.../22` blocked forever and always hit
-      # the timeout even against a perfectly healthy nix-cache, because
-      # sshd sends its banner and then holds the connection open waiting
-      # for the client to speak next -- `cat` never sees EOF, so this
-      # check reported "unreachable" unconditionally, 100% of the time,
-      # regardless of whether the remote builder was actually up.
-      if timeout 3 bash -c "exec 3<>/dev/tcp/${NIX_CACHE_HOST}/22" 2>/dev/null; then
-        builder_up=1
-        break
-      fi
-      [[ "$attempt" -lt 3 ]] && sleep 1
-    done
-    if [[ "$builder_up" -eq 0 ]]; then
-      echo "nix-cache's SSH remote builder (nixremote@${NIX_CACHE_HOST}:22) is unreachable -- disabling remote builds for the rest of this run." >&2
-      NIX_OPTS=(--builders "")
-    fi
-  fi
-
-  # `printf '%q '` with a genuinely empty NIX_OPTS still runs one format
-  # pass over a missing argument and yields the literal `'' ` rather than
-  # an empty string (confirmed live) -- a subprocess that later does
-  # `eval "NIX_OPTS=(${NIX_EXTRA_OPTS})"` (the branch above, for e.g.
-  # sync-host-keys.sh reusing this process's decision) would then rebuild
-  # a 1-element array holding an empty string instead of a 0-element
-  # array, and `nix-shell "${NIX_OPTS[@]}" -p <pkg>` chokes on that stray
-  # element as a bogus positional argument. Guard the empty case
-  # explicitly so nix-cache being reachable (NIX_OPTS legitimately empty)
-  # round-trips as truly empty instead.
-  if [[ ${#NIX_OPTS[@]} -gt 0 ]]; then
-    printf -v NIX_EXTRA_OPTS '%q ' "${NIX_OPTS[@]}"
-  else
-    NIX_EXTRA_OPTS=""
+  elif ! timeout 3 bash -c "cat < /dev/tcp/${NIX_CACHE_HOST}/22" >/dev/null 2>&1; then
+    echo "nix-cache's SSH remote builder (nixremote@${NIX_CACHE_HOST}:22) is unreachable -- disabling remote builds for the rest of this run." >&2
+    NIX_OPTS=(--builders "")
   fi
+  printf -v NIX_EXTRA_OPTS '%q ' "${NIX_OPTS[@]}"
   export NIX_EXTRA_OPTS
 }