build: align the builder image with the go.mod toolchain

go.mod moved to 1.26.5 but the docker builder stayed on golang:1.25-alpine
with GOTOOLCHAIN=local, so the tag-triggered image build refused to start.

Author: CMGS

Dockerfile

@@ -1,4 +1,4 @@
-FROM --platform=$BUILDPLATFORM golang:1.25-alpine AS build
+FROM --platform=$BUILDPLATFORM golang:1.26.5-alpine AS build
 ARG TARGETOS
 ARG TARGETARCH
 ARG VERSION=dev