ci: install docker buildx plugin for gitea release build

Debian docker.io ships no buildx plugin, so the registry-cache

build failed when the runner CLI fell back to it. Install

Docker's official CLI plus buildx plugin instead.

.gitea/workflows/release.yml

@@ -31,15 +31,31 @@ jobs:
             echo "image=registry.ginnoir.com/ginnoir/famapp"
           } >> "$GITHUB_OUTPUT"
 
-      - name: Ensure docker CLI
+      - name: Ensure docker CLI + buildx
         run: |
-          if command -v docker >/dev/null 2>&1; then
-            echo "docker already available"
+          # Debian's docker.io package ships no buildx plugin, which the
+          # registry-cache build below requires. Install Docker's official
+          # CLI + buildx plugin so the build works regardless of what the
+          # runner image happens to provide.
+          if docker buildx version >/dev/null 2>&1; then
+            echo "docker + buildx already available"
             docker version
+            docker buildx version
             exit 0
           fi
-          apt-get update -qq && apt-get install -y -qq docker.io
+
+          export DEBIAN_FRONTEND=noninteractive
+          apt-get update -qq
+          apt-get install -y -qq ca-certificates curl gnupg
+          install -m 0755 -d /etc/apt/keyrings
+          curl -fsSL https://download.docker.com/linux/debian/gpg -o /etc/apt/keyrings/docker.asc
+          chmod a+r /etc/apt/keyrings/docker.asc
+          echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian $(. /etc/os-release && echo "$VERSION_CODENAME") stable" \
+            > /etc/apt/sources.list.d/docker.list
+          apt-get update -qq
+          apt-get install -y -qq docker-ce-cli docker-buildx-plugin
           docker version
+          docker buildx version
 
       - name: Login to registry
         run: |