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>

Dockerfile

@@ -1,10 +1,7 @@
-FROM alpine:3.21 AS builder
-RUN apk add --no-cache curl xz
-WORKDIR /zig
-RUN curl -fsSL https://ziglang.org/download/0.15.2/zig-x86_64-linux-0.15.2.tar.xz | tar xJ --strip-components=1
+FROM kassany/alpine-ziglang:0.15.1 AS builder
 WORKDIR /app
 COPY . .
-RUN /zig/zig build -Doptimize=ReleaseSafe
+RUN zig build -Doptimize=ReleaseSafe
 
 FROM alpine:3.21
 RUN apk add --no-cache ffmpeg curl ca-certificates