Add Gitea Docker build/push workflow

Mirrors wisp's docker.yml: builds and pushes on push to main, image name
derived from the Gitea server/repo path.

.gitea/workflows/docker.yml

@@ -0,0 +1,53 @@
+name: Docker
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Derive image name and tags
+        id: meta
+        shell: bash
+        run: |
+          set -euo pipefail
+          SERVER_URL="${GITHUB_SERVER_URL:-${GITEA_SERVER_URL:-}}"
+          REPO="${GITHUB_REPOSITORY:-${GITEA_REPOSITORY:-}}"
+          SHA="${GITHUB_SHA:-${GITEA_SHA:-}}"
+          if [[ -z "${SERVER_URL}" || -z "${REPO}" || -z "${SHA}" ]]; then
+            echo "Missing SERVER_URL/REPO/SHA env vars." >&2; exit 1
+          fi
+          HOST=$(echo "${SERVER_URL}" | sed 's|https://||;s|http://||')
+          IMAGE=$(echo "${HOST}/${REPO}" | tr '[:upper:]' '[:lower:]')
+          SHORT_SHA=$(echo "${SHA}" | cut -c1-7)
+          echo "host=${HOST}" >> "$GITHUB_OUTPUT"
+          echo "image=${IMAGE}" >> "$GITHUB_OUTPUT"
+          echo "short_sha=${SHORT_SHA}" >> "$GITHUB_OUTPUT"
+
+      - name: Log in to Gitea container registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ steps.meta.outputs.host }}
+          username: ${{ github.actor }}
+          # Create a Gitea PAT with packages:write scope and add it as a
+          # repository secret named TKNTKN (Settings → Secrets)
+          password: ${{ secrets.TKNTKN }}
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile
+          push: true
+          tags: |
+            ${{ steps.meta.outputs.image }}:latest
+            ${{ steps.meta.outputs.image }}:${{ steps.meta.outputs.short_sha }}