ci: align pipeline on web/mydl pattern

Refactor the 4 trigger workflows to call a shared _ci-common.yaml
(hadolint + build-test), like web/mydl:

- pr.yaml       -> PR checks via _ci-common (concurrency group, dispatch)
- main.yaml     -> CI + cached load-build + git semver tag (PAT)
- tag.yaml      -> CI + build/push :vX.Y.Z and :vX.Y.Z-latest
- cron.yaml     -> nightly rebuild of :vX.Y.Z-latest
- _ci-common.yaml -> hadolint (step-level continue-on-error) + build-test
                     with GHA cache, load:true and tests/test.sh actually run
                     (real HTTP smoke tests - they existed but were never
                     invoked by any workflow)

Fixes: main.yaml tag job had a corrupted remote URL line; tests never ran
on any trigger path.

.gitea/workflows/cron.yaml

@@ -1,52 +1,46 @@
 name: Nightly Rebuild
-
 on:
   schedule:
     - cron: '0 0 * * *'
-
 jobs:
-  hadolint:
-    runs-on: ubuntu-latest
-    continue-on-error: true
-    steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
-      - uses: hadolint/hadolint-action@06be81baf89a55ffd0e24b8f04a4185738dd3387 # v3.5.0
-        with:
-          dockerfile: Dockerfile
-
-  test:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
-      - uses: docker/setup-buildx-action@f87e5991a6d7451dcb8d9637bfbc97413f497069 # v4
-      - run: docker build -t ci-image:${{ github.sha }} .
-
+  ci:
+    uses: ./.gitea/workflows/_ci-common.yaml
+    secrets: inherit
   build-push:
-    needs: [test]
+    needs: [ci]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
         with:
           fetch-depth: 0
-      - uses: docker/setup-buildx-action@f87e5991a6d7451dcb8d9637bfbc97413f497069 # v4
-      - uses: docker/login-action@dbcb813823bdd20940b903addbd779551569679f # v4
+      - 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" >> $GITHUB_OUTPUT
       - id: meta
+        name: Docker metadata
         uses: docker/metadata-action@dc802804100637a589fabce1cb79ff13a1411302 # v6
         with:
           images: jcabillot/opencode-openchamber
           tags: |
             type=raw,value=${{ steps.get-latest-tag.outputs.tag }}-latest,enable=${{ steps.get-latest-tag.outputs.tag != '' }}
-      - uses: docker/build-push-action@c3c9e263c25d99ce0380d002d59b67737d91b0dc # v7.4.0
+      - name: Build and push
+        uses: docker/build-push-action@c3c9e263c25d99ce0380d002d59b67737d91b0dc # v7.4.0
         with:
           context: .
+          file: 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