Add Gitea Actions CI pipeline for Docker image builds

Triggers on tag push or manual dispatch. Builds the image using
build/Dockerfile and pushes to the Gitea container registry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -0,0 +1,35 @@
+name: Build and Push
+
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Image tag'
+        required: true
+        default: 'latest'
+  push:
+    tags:
+      - '*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Login to Gitea registry
+        run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login -u ${{ github.actor }} --password-stdin gitea.home.hrajfrisbee.cz
+
+      - name: Build and push
+        run: |
+          TAG=${{ github.ref_name }}
+          if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
+            TAG=${{ inputs.tag }}
+          fi
+          IMAGE=gitea.home.hrajfrisbee.cz/${{ github.repository }}:$TAG
+          docker build -f build/Dockerfile -t $IMAGE .
+          docker push $IMAGE