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/_ci-common.yaml

@@ -0,0 +1,34 @@
+name: CI Shared Jobs
+
+on:
+  workflow_call:
+
+jobs:
+  hadolint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+      - uses: hadolint/hadolint-action@06be81baf89a55ffd0e24b8f04a4185738dd3387 # v3.5.0
+        with:
+          dockerfile: Dockerfile
+          failure-threshold: warning
+
+  build-test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@f87e5991a6d7451dcb8d9637bfbc97413f497069 # v4
+      - name: Build (cached)
+        uses: docker/build-push-action@c3c9e263c25d99ce0380d002d59b67737d91b0dc # v7.4.0
+        with:
+          context: .
+          file: Dockerfile
+          push: false
+          load: true
+          tags: ci-image:${{ github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+      - name: Test
+        run: bash tests/test.sh ci-image:${{ github.sha }}