fix(s6): make web services wait for their config oneshots to fix root-mode startup race

When a container built on the s6 images runs as root, php-fpm and the web
server (nginx/apache2) are brought up in parallel with the entrypoint
oneshots that configure them, because the long-running services have no
dependency on those oneshots.

As root this races:
- php-fpm reads its pool before `5-fpm-pool-user` appends `user`/`group`,
  failing with "ALERT: [pool www] user has not been defined" ->
  "ERROR: FPM initialization failed".
- the web server starts before `10-init-webserver-config` renders its
  config (e.g. nginx: open() "/etc/nginx/nginx.conf" failed).

s6 restarts the crashed services so the container eventually recovers, which
is why the failure is intermittent and hard to reproduce (see discussion
#425), but it emits alarming errors, slows startup, and leaves a brief window
with no service.

docker-php-serversideup-s6-init now adds a dependency from each web service to
the entrypoint oneshot that configures it, appending to the existing flat
`dependencies` file. The oneshots are chained in alphabetical order, so
depending on one transitively waits for all earlier ones (php-fpm ->
5-fpm-pool-user; nginx/apache2 -> 10-init-webserver-config). Entries are
de-duplicated and appended newline-safely (nginx's shipped `dependencies` has
no trailing newline). Dependencies are only added when both the service and
the oneshot exist, so cli/fpm/frankenphp images and images that remove a
script are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Lorenzo Rogai

src/s6/usr/local/bin/docker-php-serversideup-s6-init

@@ -77,4 +77,37 @@ for file in "$ENTRYPOINT_DIR"/*.sh; do
     echo "Skipping ${script_name} because it already exists at ${S6_HOME}/scripts/${script_name}"
   fi
 
-done
+done
+
+# Make the long-running services wait for the entrypoint oneshots that configure
+# them. When the container runs as root, php-fpm and the web server otherwise
+# start in parallel with these oneshots and can lose the race: php-fpm reads the
+# pool before "5-fpm-pool-user" adds "user = www-data" (ALERT: [pool www] user
+# has not been defined -> FPM initialization failed), and the web server starts
+# before "10-init-webserver-config" renders its config. s6 restarts the crashed
+# services so the container recovers, but it produces alarming errors, a slower
+# start, and a brief window with no service. The entrypoint oneshots are chained
+# in alphabetical order, so depending on one transitively waits for all earlier
+# ones. Each dependency is only added when both the service and the oneshot exist.
+add_startup_dependency() {
+  # $1 = long-running service that must wait, $2 = entrypoint oneshot it needs
+  service_dir="${S6_HOME}/s6-rc.d/$1"
+  [ -d "$service_dir" ] && [ -d "${S6_HOME}/s6-rc.d/$2" ] || return 0
+
+  dependencies_file="${service_dir}/dependencies"
+  [ -e "$dependencies_file" ] || : > "$dependencies_file"
+
+  # Skip if the dependency is already declared
+  grep -qxF "$2" "$dependencies_file" 2>/dev/null && return 0
+
+  # Ensure existing content ends with a newline before appending (some shipped
+  # dependency files, e.g. nginx's, have no trailing newline)
+  if [ -s "$dependencies_file" ] && [ -n "$(tail -c 1 "$dependencies_file")" ]; then
+    printf '\n' >> "$dependencies_file"
+  fi
+  printf '%s\n' "$2" >> "$dependencies_file"
+}
+
+add_startup_dependency php-fpm 5-fpm-pool-user
+add_startup_dependency nginx 10-init-webserver-config
+add_startup_dependency apache2 10-init-webserver-config