ci: auto-sync Docker Hub overview via peter-evans/dockerhub-description (#360)

Add a workflow that publishes DOCKERHUB.md as the snapotter/snapotter repository overview on Docker Hub, on pushes to main that touch DOCKERHUB.md and via manual dispatch. Commit DOCKERHUB.md (previously gitignored) so CI can read it, and refresh its tool counts to 240 to match the README. Auth uses the DOCKER_HUB_README secret (Docker Hub account password) and the existing DOCKERHUB_USERNAME.

.github/workflows/dockerhub-description.yml

@@ -0,0 +1,30 @@
+name: Sync Docker Hub description
+
+# Publishes DOCKERHUB.md as the snapotter/snapotter repository overview on Docker Hub
+# when it changes on main (or on manual dispatch). Secret DOCKER_HUB_README holds the
+# Docker Hub account password; DOCKERHUB_USERNAME already exists (used by release.yml).
+
+on:
+  push:
+    branches: [main]
+    paths: ["DOCKERHUB.md"]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: dockerhub-description
+  cancel-in-progress: true
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: peter-evans/dockerhub-description@1b9a80c056b620d92cedb9d9b5a223409c68ddfa # v5.0.0
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKER_HUB_README }}
+          repository: snapotter/snapotter
+          readme-filepath: ./DOCKERHUB.md