Update ubuntu Docker tag to v26

roles/custom/matrix-bridge-rustpush/templates/Dockerfile.j2

@@ -6,7 +6,7 @@ SPDX-License-Identifier: AGPL-3.0-or-later
 #}
 
 # ── Stage 1: builder ─────────────────────────────────────────────────────────
-FROM ubuntu:24.04 AS builder
+FROM ubuntu:26.04 AS builder
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -80,7 +80,7 @@ RUN BUILD_TIME=$(date -u +%Y-%m-%dT%H:%M:%SZ) && \
     ./cmd/matrix-rustpush/
 
 # ── Stage 2: runtime ─────────────────────────────────────────────────────────
-FROM ubuntu:24.04
+FROM ubuntu:26.04
 
 ENV DEBIAN_FRONTEND=noninteractive