feat: add Ubuntu 26.04 (Resolute) support

Add Dockerfile, package script, and CI/build configuration for ubuntu-2604 across all build targets.

builder/docker-compose.yml

@@ -35,6 +35,18 @@ services:
     image: r-builds:ubuntu-2404
     volumes:
       - /tmp/ubuntu-2404:/tmp/output/ubuntu-2404
+  ubuntu-2604:
+    command: ./build.sh
+    environment:
+      - R_VERSION=${R_VERSION}
+      - R_INSTALL_PATH=${R_INSTALL_PATH}
+      - LOCAL_STORE=/tmp/output
+    build:
+      context: .
+      dockerfile: Dockerfile.ubuntu-2604
+    image: r-builds:ubuntu-2604
+    volumes:
+      - /tmp/ubuntu-2604:/tmp/output/ubuntu-2604
   alpine-320:
     command: ./build.sh
     environment: