Build the image with Go 1.26

The Prometheus client and the golang.org/x updates raised go.mod to Go 1.26,
so the Docker build with golang:1.25-alpine failed in go mod download.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -3,7 +3,7 @@ ARG FRONTEND_IMAGE=gitea.sanplex.xyz/sansan/musicfrontend:latest
 
 # Stage 1: Build the backend on the build machine and cross-compile for the target platform,
 # so multi-platform builds need no emulation.
-FROM --platform=$BUILDPLATFORM golang:1.25-alpine AS build
+FROM --platform=$BUILDPLATFORM golang:1.26-alpine AS build
 ARG TARGETOS TARGETARCH
 WORKDIR /app
 COPY go.mod go.sum ./