fix: make docker build/push work with Gitea

- Replace type=gha cache (broken on Gitea: unparseable runtime token)
  with type=registry cache using image-manifest/oci-mediatypes options
  that the Gitea registry accepts.
- Push with a Gitea PAT (REGISTRY_TOKEN) since the auto job token
  cannot publish OCI images to the package repo.
- Add registry login to the CI docker smoke-test job.

.gitea/workflows/ci.yml

@@ -14,6 +14,7 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: "3.13"
+          cache: pip
       - run: pip install ruff
       - run: ruff check src/
       - run: ruff format --check src/
@@ -36,8 +37,14 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        with:
+          registry: git.soconnor.dev
+          username: ${{ gitea.repository_owner }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
       - uses: docker/build-push-action@v6
         with:
           context: .
           push: false
           tags: poke-mail:test
+          cache-from: type=registry,ref=git.soconnor.dev/soconnor/poke-mail:buildcache