fix(cd): Add new cd steps with buildx for lambda arm/amd images (#37)

# Problem #
Arm support needs lambda image to be updated for both architectures

# Solution #
Update Lambda cd steps to use buildx for both images to be tagged
together

Author: adityasadalage

.github/workflows/cd-dgraph-lambda.yml

@@ -0,0 +1,31 @@
+name: cd-dgraph-lambda
+on: workflow_dispatch
+jobs:
+  dgraph-lambda-build:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set dgraph-lambda Release Version
+        run: |
+          #!/bin/bash
+          GIT_BRANCH_NAME=$(git rev-parse --abbrev-ref HEAD)
+          if [[ "$GIT_BRANCH_NAME" == "release/v"* ]]; 
+          then
+            echo "this is a release branch"
+          else
+            echo "this is NOT a release branch"
+            exit 1
+          fi
+          DGRAPH_LAMBDA_RELEASE_VERSION=$(git rev-parse --abbrev-ref HEAD | sed  's/release\///')
+          echo "making a new release for dgraph "$DGRAPH_LAMBDA_RELEASE_VERSION
+          echo "DGRAPH_LAMBDA_RELEASE_VERSION=$DGRAPH_LAMBDA_RELEASE_VERSION" >> $GITHUB_ENV
+      - name: Login to DockerHub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_PASSWORD_TOKEN }}
+      - name: Push dgraph-lambda images
+        run: |
+          docker buildx create --name builder --driver docker-container
+          docker buildx use builder
+          docker buildx build -t dgraph/dgraph-lambda:${{ env.DGRAPH_LAMBDA_RELEASE_VERSION }} --push --platform=linux/arm64,linux/amd64 .