Publish install manifests to GHCR and DockerHub as OCI artifacts

Add workflow to build and push the install manifests to:
- ghcr.io/fluxcd/flux-manifests
- docker.io/fluxcd/flux-manifests
The OCI artifacts are signed with Cosign and GitHub OIDC (keyless).
The manifests pushed to GHCR have the container images set to ghcr.io/fluxcd/<controller-name> while the manifests pushed to DockerHub have the controller images set to docker.io/fluxcd/<controller-name>.

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

.github/workflows/release-manifests.yml

@@ -0,0 +1,73 @@
+name: release-manifests
+on:
+  release:
+    types: [published]
+  workflow_dispatch:
+
+permissions:
+  id-token: write # needed for keyless signing
+  packages: write # needed for ghcr access
+
+jobs:
+  build-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Setup Kustomize
+        uses: fluxcd/pkg/actions/kustomize@main
+      - name: Setup Flux CLI
+        uses: ./action/
+      - name: Prepare
+        id: prep
+        run: |
+          VERSION=$(flux version --client | awk '{ print $NF }')
+          echo ::set-output name=VERSION::${VERSION}
+      - name: Login to GHCR
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: fluxcdbot
+          password: ${{ secrets.GHCR_TOKEN }}
+      - name: Login to DockerHub
+        uses: docker/login-action@v2
+        with:
+          username: fluxcdbot
+          password: ${{ secrets.DOCKER_FLUXCD_PASSWORD }}
+      - name: Push manifests to GHCR
+        run: |
+          mkdir -p ./ghcr.io/flux-system
+          flux install --registry=ghcr.io/fluxcd \
+          --components-extra=image-reflector-controller,image-automation-controller \
+          --export > ./ghcr.io/flux-system/gotk-components.yaml
+          
+          cd ./ghcr.io && flux push artifact \
+          oci://ghcr.io/fluxcd/flux-manifests:${{ steps.prep.outputs.VERSION }} \
+          --path="./flux-system" \
+          --source=${{ github.repositoryUrl }} \
+          --revision="${{ github.ref_name }}/${{ github.sha }}"
+      - name: Push manifests to DockerHub
+        run: |
+          mkdir -p ./docker.io/flux-system
+          flux install --registry=docker.io/fluxcd \
+          --components-extra=image-reflector-controller,image-automation-controller \
+          --export > ./docker.io/flux-system/gotk-components.yaml
+          
+          cd ./docker.io && flux push artifact \
+          oci://docker.io/fluxcd/flux-manifests:${{ steps.prep.outputs.VERSION }} \
+          --path="./flux-system" \
+          --source=${{ github.repositoryUrl }} \
+          --revision="${{ github.ref_name }}/${{ github.sha }}"
+      - uses: sigstore/cosign-installer@main
+      - name: Sign manifests
+        env:
+          COSIGN_EXPERIMENTAL: 1
+        run: |
+          cosign sign ghcr.io/fluxcd/flux-manifests:${{ steps.prep.outputs.VERSION }}
+          cosign sign docker.io/fluxcd/flux-manifests:${{ steps.prep.outputs.VERSION }}
+      - name: Tag manifests
+        run: |
+          flux tag artifact oci://ghcr.io/fluxcd/flux-manifests:${{ steps.prep.outputs.VERSION }} \
+          --tag latest
+
+          flux tag artifact oci://docker.io/fluxcd/flux-manifests:${{ steps.prep.outputs.VERSION }} \
+          --tag latest