Update docker/dockerfile Docker tag to v1.9 (#2444)

| datasource | package           | from | to  |
| ---------- | ----------------- | ---- | --- |
| docker     | docker/dockerfile | 1.8  | 1.9 |

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

build/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.8
+# syntax=docker/dockerfile:1.9
 FROM golang:1.23 AS builder
 
 WORKDIR /go/src/github.com/nginxinc/nginx-gateway-fabric

build/Dockerfile.nginx

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.8
+# syntax=docker/dockerfile:1.9
 FROM nginx:1.27.1-alpine-otel
 
 ARG NJS_DIR

build/Dockerfile.nginxplus

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.8
+# syntax=docker/dockerfile:1.9
 FROM scratch AS nginx-files
 
 # the following links can be replaced with local files if needed, i.e. ADD --chown=101:1001 <local_file> <container_file>

debug/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.8
+# syntax=docker/dockerfile:1.9
 # This Dockerfile builds an image with the dlv debugger. See the debugging guide in the developer docs for details
 # on how to use it.
 FROM golang:1.23-alpine AS builder

tests/Dockerfile

@@ -1,3 +1,3 @@
-# syntax=docker/dockerfile:1.8
+# syntax=docker/dockerfile:1.9
 # this is here so we can grab the latest version of kind and have dependabot keep it up to date
 FROM kindest/node:v1.31.0

tests/conformance/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.8
+# syntax=docker/dockerfile:1.9
 FROM golang:1.23
 
 WORKDIR /go/src/github.com/nginxinc/nginx-gateway-fabric/tests/conformance