github: publish upstream container to ghcr

At some point we stopped building and pushing the container to ghcr to
focus on having only Konflux builds and a single source of the
upstream (though, more like "midstream") container on quay.io.  However,
Konflux builds are not entirely reliable and are inaccessible to
external users and contributors.  So while the container itself is
publicly available under the quay.io/centos-bootc namespace,
investigating (and solving) build failures requires contacting people
with access to the private job logs.

Re-enabling ghcr builds allows us (and practically anyone) to easily
investigate and solve build failures and gives us a reliable upstream
container available shortly after every PR is merged, making
troubleshooting against the latest version a lot easier.

This reverts commit 62dda27.

See also discussion in #1000.

Author: achilleas-k

.github/workflows/build.yaml

@@ -3,14 +3,19 @@ name: Build containers
 on:
   pull_request:
     branches: [main]
+  workflow_dispatch:
   # for merge queue
   merge_group:
+  push:
+    branches: [main]
 
 env:
+  REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository }}
 
 permissions:
   contents: read
+  packages: write
 
 jobs:
   build:
@@ -25,3 +30,19 @@ jobs:
           image: ${{ env.IMAGE_NAME }}
           tags: "latest"
           containerfiles: Containerfile
+
+      - name: Log in to the Container registry
+        if: ${{ (github.event_name == 'workflow_dispatch' || github.event_name == 'push') && github.ref == 'refs/heads/main' }}
+        uses: redhat-actions/podman-login@v1
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Push to GitHub Container Repository
+        if: ${{ (github.event_name == 'workflow_dispatch' || github.event_name == 'push') && github.ref == 'refs/heads/main' }}
+        uses: redhat-actions/push-to-registry@v2
+        with:
+          image: ${{ env.IMAGE_NAME }}
+          tags: "latest"
+          registry: ${{ env.REGISTRY }}