chore: add `syntax` parser directive to Dockerfile

Dockerfile.alpine

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 FROM docker.io/nginxproxy/docker-gen:0.14.5 AS docker-gen
 
 FROM docker.io/nginxproxy/forego:0.18.2 AS forego

Dockerfile.debian

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 FROM docker.io/nginxproxy/docker-gen:0.14.5-debian AS docker-gen
 
 FROM docker.io/nginxproxy/forego:0.18.2-debian AS forego

test/requirements/web/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Docker Image running one (or multiple) webservers listening on all given ports from WEB_PORTS environment variable
 
 FROM python:3-alpine