chore: add PKG_KERNEL arg to customize used kernel

Related to #7612

Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

Dockerfile

@@ -4,6 +4,7 @@
 
 ARG TOOLS
 ARG PKGS
+ARG PKG_KERNEL
 ARG EXTRAS
 ARG INSTALLER_ARCH
 
@@ -77,9 +78,9 @@ FROM --platform=arm64 ghcr.io/siderolabs/util-linux:${PKGS} AS pkg-util-linux-ar
 FROM --platform=amd64 ghcr.io/siderolabs/kmod:${PKGS} AS pkg-kmod-amd64
 FROM --platform=arm64 ghcr.io/siderolabs/kmod:${PKGS} AS pkg-kmod-arm64
 
-FROM ghcr.io/siderolabs/kernel:${PKGS} AS pkg-kernel
-FROM --platform=amd64 ghcr.io/siderolabs/kernel:${PKGS} AS pkg-kernel-amd64
-FROM --platform=arm64 ghcr.io/siderolabs/kernel:${PKGS} AS pkg-kernel-arm64
+FROM ${PKG_KERNEL} AS pkg-kernel
+FROM --platform=amd64 ${PKG_KERNEL} AS pkg-kernel-amd64
+FROM --platform=arm64 ${PKG_KERNEL} AS pkg-kernel-arm64
 
 FROM --platform=arm64 ghcr.io/siderolabs/u-boot:${PKGS} AS pkg-u-boot-arm64
 FROM --platform=arm64 ghcr.io/siderolabs/raspberrypi-firmware:${PKGS} AS pkg-raspberrypi-firmware-arm64

Makefile

@@ -17,6 +17,7 @@ CLOUD_IMAGES_EXTRA_ARGS ?= ""
 ARTIFACTS := _out
 TOOLS ?= ghcr.io/siderolabs/tools:v1.6.0-alpha.0-1-gfa388de
 PKGS ?= v1.6.0-alpha.0-12-g518c441
+PKG_KERNEL ?= ghcr.io/siderolabs/kernel:$(PKGS)
 EXTRAS ?= v1.6.0-alpha.0
 # renovate: datasource=github-tags depName=golang/go
 GO_VERSION ?= 1.21
@@ -108,6 +109,7 @@ COMMON_ARGS += --platform=$(PLATFORM)
 COMMON_ARGS += --push=$(PUSH)
 COMMON_ARGS += --build-arg=TOOLS=$(TOOLS)
 COMMON_ARGS += --build-arg=PKGS=$(PKGS)
+COMMON_ARGS += --build-arg=PKG_KERNEL=$(PKG_KERNEL)
 COMMON_ARGS += --build-arg=EXTRAS=$(EXTRAS)
 COMMON_ARGS += --build-arg=GOFUMPT_VERSION=$(GOFUMPT_VERSION)
 COMMON_ARGS += --build-arg=GOIMPORTS_VERSION=$(GOIMPORTS_VERSION)