manibuilder/alpine: Install Clang for select container

This adds Clang 19 to our build tests.

Change-Id: I5e3648b759a5c9604621c0436036bc91f594960b
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/367
diff --git a/util/manibuilder/Makefile.env b/util/manibuilder/Makefile.env
index 39aabde..b6af5ca 100644
--- a/util/manibuilder/Makefile.env
+++ b/util/manibuilder/Makefile.env
@@ -33,5 +33,6 @@
 CLANG_TAGS := \
 	debian-debootstrap\:armhf-bullseye debian-debootstrap\:i386-bullseye \
 	arm64v8/debian\:bookworm amd64/debian\:bookworm \
+	amd64/alpine\:3.21 \
 
 $(addsuffix -build,$(CLANG_TAGS)): DOCKER_BUILD_ARGS+=--build-arg ADDITIONAL_PACKAGES=clang