ci: align pipeline on web/mydl pattern

Split the monolithic docker-build/release/test workflows into the shared
_ci-common.yaml + trigger-specific files used on web/mydl:

- pr.yaml       -> PR checks (hadolint + build smoke test)
- main.yaml     -> CI + local build + git semver tag (PAT, triggers tag.yaml)
- tag.yaml      -> CI + build/push :vX.Y.Z and :vX.Y.Z-latest (sole Docker push)
- cron.yaml     -> nightly rebuild of :vX.Y.Z-latest
- _ci-common.yaml -> hadolint + build-test with docker smoke and GHA cache

Docker pushes to :latest on every merge are replaced by versioned,
tested tags. The tag job uses SA_TOKEN_ACTION_PUSH_TAGS (a PAT) so the
tag push actually triggers tag.yaml - GITHUB_TOKEN would not.

.gitea/workflows/cron.yaml

@@ -0,0 +1,46 @@
+name: Nightly Rebuild
+on:
+  schedule:
+    - cron: '0 0 * * *'
+jobs:
+  ci:
+    uses: ./.gitea/workflows/_ci-common.yaml
+    secrets: inherit
+  build-push:
+    needs: [ci]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+        with:
+          fetch-depth: 0
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@f87e5991a6d7451dcb8d9637bfbc97413f497069 # v4
+      - name: Login to Docker Hub
+        uses: docker/login-action@dbcb813823bdd20940b903addbd779551569679f # v4
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - id: get-latest-tag
+        name: Get latest tag
+        run: |
+          TAG=$(git describe --tags --abbrev=0 2>/dev/null || echo "")
+          echo "tag=$TAG" >> $GITHUB_OUTPUT
+      - id: meta
+        name: Docker metadata
+        uses: docker/metadata-action@dc802804100637a589fabce1cb79ff13a1411302 # v6
+        with:
+          images: jcabillot/mcp-ics
+          tags: |
+            type=raw,value=${{ steps.get-latest-tag.outputs.tag }}-latest,enable=${{ steps.get-latest-tag.outputs.tag != '' }}
+      - name: Build and push
+        uses: docker/build-push-action@c3c9e263c25d99ce0380d002d59b67737d91b0dc # v7.4.0
+        with:
+          context: .
+          file: pkg/Dockerfile
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          pull: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max