github: build docker-container on pull-request

Also build the docker-container whan creating pull-requests. Keep the
pull-request triggered builds from pushing to the container registry.

Signed-off-by: David Bauer <mail@david-bauer.net>

.github/workflows/build-container.yml

@@ -8,6 +8,7 @@ on:
       - 'v202[0-9].[0-9].x'
     tags:
       - 'v*'
+  pull_request:
 
 env:
   REGISTRY: ghcr.io
@@ -22,7 +23,7 @@ jobs:
         uses: actions/checkout@v4
       - name: Log in to the Container registry
         uses: docker/login-action@b4bedf8053341df3b5a9f9e0f2cf4e79e27360c6
-        if: ${{ github.repository_owner == 'freifunk-gluon' }}
+        if: ${{ github.repository_owner == 'freifunk-gluon' && github.event_name == 'push' }}
         with:
           registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
@@ -36,6 +37,6 @@ jobs:
         uses: docker/build-push-action@4c1b68d83ad20cc1a09620ca477d5bbbb5fa14d0
         with:
           context: ./contrib/docker
-          push: ${{ github.repository_owner == 'freifunk-gluon' }}
+          push: ${{ github.repository_owner == 'freifunk-gluon' && github.event_name == 'push' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}