Fix Dockerfile-build: use Go 1.26 and add the cluster package

go.mod requires Go 1.26.0, but the builder image was golang:1.25 (with
GOTOOLCHAIN=local), and the server now imports the cluster package.

Fixes #1962

Dockerfile-build

@@ -1,4 +1,4 @@
-FROM golang:1.25-bookworm AS builder
+FROM golang:1.26-bookworm AS builder
 
 ARG VERSION=dev
 ARG COMMIT=unknown
@@ -52,6 +52,7 @@ ADD ./action ./action
 ADD ./ban ./ban
 ADD ./metrics ./metrics
 ADD ./twilio ./twilio
+ADD ./cluster ./cluster
 ADD ./template/gotext ./template/gotext
 
 RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache/go-build make VERSION=$VERSION COMMIT=$COMMIT cli-linux-server

docs/releases.md

@@ -2126,6 +2126,7 @@ and the [ntfy Android app](https://github.com/binwiederhier/ntfy-android/release
 
 **Bug fixes + maintenance:**
 
+* Fix `Dockerfile-build` failing to build: use Go 1.26 and add the `cluster` package ([#1962](https://github.com/binwiederhier/ntfy/issues/1962), thanks to [@ElectricTea](https://github.com/ElectricTea) for reporting)
 * Warn at startup if `attachment-expiry-duration` is longer than `cache-duration`, since attachments never outlive their message ([#1053](https://github.com/binwiederhier/ntfy/issues/1053), thanks to [@ctschach](https://github.com/ctschach) for reporting)
 * Fix PostgreSQL memory growth after v2.29.0: write message batches with one fixed `INSERT` for any batch size ([#2011](https://github.com/binwiederhier/ntfy/pull/2011))