fix(docker): bump build image to golang:1.26.6

The release image build failed after go.mod moved to 1.26.6: the Dockerfile
pinned golang:1.26.5 and the image runs GOTOOLCHAIN=local, so `go mod download`
refused the newer directive. CI didn't catch it — it resolves the toolchain
from go.mod and downloads it; only the Docker build has a fixed base.

Author: AppsGanin

Dockerfile

@@ -11,7 +11,7 @@ RUN npm run build
 
 # 2) Build the Go binary (embeds the SPA from /web/dist). CGO is off, so we
 #    cross-compile from the build platform to $TARGETARCH instead of emulating.
-FROM --platform=$BUILDPLATFORM golang:1.26.5 AS build
+FROM --platform=$BUILDPLATFORM golang:1.26.6 AS build
 ARG TARGETARCH
 WORKDIR /src
 COPY go.mod go.sum ./