ci: enable docker build & sha extraction on PRs

- Get short SHA on all events (PRs + pushes)
- Registry login on all events
- Build Docker image on all events (validate Dockerfile on PRs)
- Push only on push/workflow_dispatch (not PRs)
- Prune images on all events

This ensures PR builds verify Docker image builds successfully

.gitea/workflows/build.yaml

@@ -36,12 +36,10 @@ jobs:
         run: cargo clippy --all --all-targets -- -D warnings 2>&1 | tail -50 || true
 
       - name: Get short SHA
-        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         id: sha
         run: echo "short_sha=$(git rev-parse --short HEAD)" >> $GITHUB_OUTPUT
 
       - name: Registry login
-        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         run: |
           echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" \
             --username "${REGISTRY_USER}" --password-stdin
@@ -50,7 +48,6 @@ jobs:
           REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
 
       - name: Build Docker image
-        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         run: |
           docker build --no-cache --progress=plain \
             -t "${IMAGE}:${{ steps.sha.outputs.short_sha }}" \
@@ -65,5 +62,4 @@ jobs:
           echo "✓ Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
 
       - name: Prune unused images
-        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         run: docker image prune -a --force 2>&1 | tail -3 || true