Generate image pull secret at bootstrap

Add an optional flag called `--registry-creds` to the bootstrap
command for generating an image pull secret for container images
stored in private registries.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

.github/workflows/e2e-bootstrap.yaml

@@ -53,16 +53,22 @@ jobs:
         run: |
           /tmp/flux bootstrap github --manifests ./manifests/install/ \
           --owner=fluxcd-testing \
+          --image-pull-secret=ghcr-auth \
+          --registry-creds=fluxcd:$GITHUB_TOKEN \
           --repository=${{ steps.vars.outputs.test_repo_name }} \
           --branch=main \
           --path=test-cluster \
           --team=team-z
         env:
           GITHUB_TOKEN: ${{ secrets.GITPROVIDER_BOT_TOKEN }}
+      - name: verify image pull secret
+        run: |
+          kubectl -n flux-system get secret ghcr-auth | grep dockerconfigjson
       - name: bootstrap no-op
         run: |
           /tmp/flux bootstrap github --manifests ./manifests/install/ \
           --owner=fluxcd-testing \
+          --image-pull-secret=ghcr-auth \
           --repository=${{ steps.vars.outputs.test_repo_name }} \
           --branch=main \
           --path=test-cluster \