Fix CI: use kassany/alpine-ziglang Docker image for build

ziglang.org unreachable from cluster, use pre-built Docker image instead.
Lower minimum_zig_version to 0.15.1 for compatibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build.zig.zon

@@ -25,7 +25,7 @@
     .fingerprint = 0x4f84e6a7e054cdf3, // Changing this has security and trust implications.
     // Tracks the earliest Zig version that the package considers to be a
     // supported use case.
-    .minimum_zig_version = "0.15.2",
+    .minimum_zig_version = "0.15.1",
     // This field is optional.
     // Each dependency must either provide a `url` and `hash`, or a `path`.
     // `zig build --fetch` can be used to fetch all dependencies of a package, recursively.