🐛 Fix __format_variables returning a single space for empty input 🐛

App-breaking bug found during full runtime verification of the act_runner
cache-server feature: gitea failed to start with `Command error: unknown
command: /config/gitea/app.ini` because its `--port` argument was empty.

Root cause: `__format_variables()` ran `printf '%s\n' $input | sort -Ru |
tr '\n' ' '` unconditionally. When `$input` word-splits to zero words
(whitespace-only, e.g. no port env vars set), `printf` with a `%s` format
still runs once with a missing arg, emitting a blank line — so the
function returned a single space `" "` instead of empty. That made
`ENV_PORTS` / `WEB_SERVER_PORTS` resolve to `" "`, which made
`SERVICE_PORT` in `08-gitea.sh` become `" "` — non-empty per `[ -n ... ]`
but rendering as an empty `--port` value to `gitea web`.

- rootfs/usr/local/etc/docker/functions/entrypoint.sh: replaced
  `[ -z "$input" ] && return 0` with
  `[[ "$input" =~ [^[:space:]] ]] || return 0` so whitespace-only input
  is treated as empty before reaching the `printf` pipeline
- TODO.AI.md: logged the fix and the upstream-template-sync follow-up

Verified: rebuilt the image and confirmed `gitea will be running on port
80` / `gitea web --port 80 ...` in a full container run (previously
`--port` with no value).

rootfs/usr/local/etc/docker/functions/entrypoint.sh

@@ -186,7 +186,7 @@ __get_pid() {
 # - - - - - - - - - - - - - - - - - - - - - - - - -
 __format_variables() {
   local input="${*//,/ }"
-  [ -z "$input" ] && return 0
+  [[ "$input" =~ [^[:space:]] ]] || return 0
   printf '%s\n' $input | sort -Ru | tr '\n' ' '
 }
 # - - - - - - - - - - - - - - - - - - - - - - - - -