Add Gitea Actions image-build workflow

Distilled from the house pattern across sibling projects: tag push +
workflow_dispatch triggers, REGISTRY_TOKEN login, raw docker build/push
to gitea.home.hrajfrisbee.cz. Adds a lightweight test gate, an immutable
sha-<12> tag, :latest only on real tag pushes, and a concurrency group.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -0,0 +1,76 @@
+name: Build and Push
+
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Image tag'
+        required: true
+        default: 'latest'
+  push:
+    tags:
+      - '*'
+
+concurrency:
+  group: build-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+      - name: Vet
+        run: go vet ./...
+      - name: Build
+        run: go build ./...
+      - name: Test (short)
+        run: go test -short ./...
+
+  build:
+    needs: check
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Compute image tags
+        id: meta
+        run: |
+          if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
+            TAG="${{ inputs.tag }}"
+          else
+            TAG="${{ github.ref_name }}"
+          fi
+          echo "tag=$TAG" >> "$GITHUB_OUTPUT"
+          echo "sha=sha-$(echo '${{ github.sha }}' | cut -c1-12)" >> "$GITHUB_OUTPUT"
+
+      - name: Login to Gitea registry
+        run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login -u ${{ github.actor }} --password-stdin gitea.home.hrajfrisbee.cz
+
+      - name: Build and push
+        run: |
+          IMAGE=gitea.home.hrajfrisbee.cz/${{ github.repository }}
+          docker build \
+            --build-arg GIT_COMMIT=$(echo '${{ github.sha }}' | cut -c1-12) \
+            --label org.opencontainers.image.source=https://gitea.home.hrajfrisbee.cz/${{ github.repository }} \
+            --label org.opencontainers.image.created=$(date -u +%Y-%m-%dT%H:%M:%SZ) \
+            -t "$IMAGE:${{ steps.meta.outputs.tag }}" \
+            -t "$IMAGE:${{ steps.meta.outputs.sha }}" \
+            .
+          docker push "$IMAGE:${{ steps.meta.outputs.tag }}"
+          docker push "$IMAGE:${{ steps.meta.outputs.sha }}"
+
+      # Only real tag pushes move :latest — an ad-hoc dispatch of an old ref must not clobber it.
+      - name: Push latest (tag builds only)
+        if: github.event_name == 'push'
+        run: |
+          IMAGE=gitea.home.hrajfrisbee.cz/${{ github.repository }}
+          docker tag "$IMAGE:${{ steps.meta.outputs.tag }}" "$IMAGE:latest"
+          docker push "$IMAGE:latest"