ci: add docker build and push action

.forgejo/workflows/build-image.yml

@@ -0,0 +1,20 @@
+name: Build docker image
+on: [push]
+
+jobs:
+  build-image:
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    steps:
+      - name: Login to Forgejo docker registry
+        uses: docker/login-action@v3.0.0
+        with:
+          registry: git.pub.solar
+          username: hakkonaut
+          password: ${{ secrets.GIT_AUTH_TOKEN }}
+      - name: Build and push
+        uses: docker/build-push-action@v5.1.0
+        with:
+          push: true
+          tags: git.pub.solar/pub-solar/obs-portal:latest