Update docker/dockerfile Docker tag to v1.15

| datasource | package           | from | to   |
| ---------- | ----------------- | ---- | ---- |
| docker     | docker/dockerfile | 1.14 | 1.15 |


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

Author: renovate[bot]

Diff for: build/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.14
+# syntax=docker/dockerfile:1.15
 FROM golang:1.24 AS builder
 
 WORKDIR /go/src/github.com/nginx/nginx-gateway-fabric

Diff for: build/Dockerfile.nginx

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.14
+# syntax=docker/dockerfile:1.15
 FROM nginx:1.27.4-alpine-otel
 
 ARG NJS_DIR

Diff for: build/Dockerfile.nginxplus

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.14
+# syntax=docker/dockerfile:1.15
 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>

Diff for: debug/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.14
+# syntax=docker/dockerfile:1.15
 # 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:1.24-alpine AS builder

Diff for: tests/conformance/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.14
+# syntax=docker/dockerfile:1.15
 FROM golang:1.24
 
 WORKDIR /go/src/github.com/nginx/nginx-gateway-fabric/tests/conformance

Diff for: tests/framework/crossplane/Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.14
+# syntax=docker/dockerfile:1.15
 FROM golang:1.24-alpine AS builder
 
 WORKDIR /