.github: add workflow to build beta images

With this new workflow, developers will be able to release beta features
that are created on top of an existing release.

The workflow to create a new beta image is as follow:

1. Push a branch into Cilium's repository with the name:
   `feature/<stable-branch>/<feature-name>` where `<stable-branch>`
   represents the branch where the feature is based on and
   `<feature-name>` represents the name of the feature being released.
2. Trigger the workflow by going into [1], use the workflow from
   `feature/<stable-branch>/<feature-name>` branch and write an image
   tag name.
   The tag name should be in the format `vX.Y.Z-<feature-name>` where
   `vX.Y.Z` is the version on which the branch is built on, and
   `<feature-name>` the name of the feature.
3. Ping one of the maintainers or anyone from the cilium-build team to
   approve the build and release process of this feature.

[1] https://github.com/cilium/cilium/actions/workflows/build-images-beta.yaml

Signed-off-by: André Martins <andre@cilium.io>

.github/workflows/build-images-beta.yaml

@@ -0,0 +1,129 @@
+name: Beta Image Release Build
+
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: 'Docker Image Tag'
+        required: true
+
+permissions: read-all
+
+jobs:
+  build-and-push:
+    if: ${{ github.repository == 'cilium/cilium' }}
+    environment: release-beta-images
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        include:
+          - name: cilium
+            dockerfile: ./images/cilium/Dockerfile
+
+          - name: operator
+            dockerfile: ./images/operator/Dockerfile
+
+          - name: operator-aws
+            dockerfile: ./images/operator/Dockerfile
+
+          - name: operator-azure
+            dockerfile: ./images/operator/Dockerfile
+
+          - name: operator-alibabacloud
+            dockerfile: ./images/operator/Dockerfile
+
+          - name: operator-generic
+            dockerfile: ./images/operator/Dockerfile
+
+          - name: hubble-relay
+            dockerfile: ./images/hubble-relay/Dockerfile
+
+          - name: clustermesh-apiserver
+            dockerfile: ./images/clustermesh-apiserver/Dockerfile
+
+          - name: docker-plugin
+            dockerfile: ./images/cilium-docker-plugin/Dockerfile
+
+    steps:
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@94ab11c41e45d028884a99163086648e898eed25
+
+      - name: Login to quay.io
+        uses: docker/login-action@f054a8b539a109f9f41c372932f1ae047eff08c9
+        with:
+          registry: quay.io
+          username: ${{ secrets.QUAY_BETA_USERNAME }}
+          password: ${{ secrets.QUAY_BETA_PASSWORD }}
+
+      - name: Getting image tag
+        id: tag
+        run: |
+          echo ::set-output name=tag::${GITHUB_REF##*/}
+
+      - name: Checking if tag already exists
+        id: tag-in-repositories
+        shell: bash
+        run: |
+          if docker buildx imagetools inspect quay.io/${{ github.repository_owner }}/${{ matrix.name }}-beta:${{ github.event.inputs.tag }} &>/dev/null; then
+            echo "Tag already exists!"
+            exit 1
+          fi
+
+      - name: Checkout Source Code
+        uses: actions/checkout@ec3a7ce113134d7a93b817d10a8272cb61118579
+        with:
+          persist-credentials: false
+
+      - name: Release Build ${{ matrix.name }}
+        uses: docker/build-push-action@a66e35b9cbcf4ad0ea91ffcaf7bbad63ad9e0229
+        id: docker_build_release
+        with:
+          context: .
+          file: ${{ matrix.dockerfile }}
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: |
+            quay.io/${{ github.repository_owner }}/${{ matrix.name }}-beta:${{ github.event.inputs.tag }}
+            quay.io/${{ github.repository_owner }}/${{ matrix.name }}-ci:${{ github.sha }}
+          build-args: |
+            OPERATOR_VARIANT=${{ matrix.name }}
+
+      - name: Image Release Digest
+        shell: bash
+        run: |
+          mkdir -p image-digest/
+          echo "## ${{ matrix.name }}" > image-digest/${{ matrix.name }}.txt
+          echo "" >> image-digest/${{ matrix.name }}.txt
+          echo "\`quay.io/${{ github.repository_owner }}/${{ matrix.name }}-beta:${{ github.event.inputs.tag }}@${{ steps.docker_build_release.outputs.digest }}\`" >> image-digest/${{ matrix.name }}.txt
+          echo "\`quay.io/${{ github.repository_owner }}/${{ matrix.name }}-ci:${{ github.sha }}@${{ steps.docker_build_release.outputs.digest }}\`" >> image-digest/${{ matrix.name }}.txt
+          echo "" >> image-digest/${{ matrix.name }}.txt
+
+      # Upload artifact digests
+      - name: Upload artifact digests
+        uses: actions/upload-artifact@27121b0bdffd731efa15d66772be8dc71245d074
+        with:
+          name: image-digest ${{ matrix.name }}
+          path: image-digest
+          retention-days: 1
+
+  image-digests:
+    if: ${{ github.repository == 'cilium/cilium' }}
+    name: Display Digests
+    runs-on: ubuntu-20.04
+    needs: build-and-push
+    steps:
+      - name: Downloading Image Digests
+        shell: bash
+        run: |
+          mkdir -p image-digest/
+
+      - name: Download digests of all images built
+        uses: actions/download-artifact@3be87be14a055c47b01d3bd88f8fe02320a9bb60
+        with:
+          path: image-digest/
+
+      - name: Image Digests Output
+        shell: bash
+        run: |
+          cd image-digest/
+          find -type f | sort | xargs -d '\n' cat