fix: get digest of image

Author: ayuskauskas

.github/workflows/build_agent_container.yaml

@@ -49,6 +49,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
 
       - name: Build the agent container image
+        id: build
         run: |
           apt-get update && apt-get install -y make git
           cd agent
@@ -57,6 +58,7 @@ jobs:
           # Get the last tag and use it as the env var AGENT_VERSION if it doesn't exist use 0.0.0+{github.sha}
           export AGENT_VERSION=$(git describe --tags --abbrev=0 2>/dev/null || echo "0.0.0+${{ github.sha }}")
           make docker-build-only agent_version=${AGENT_VERSION}
+          echo "digest=$(docker inspect --format='{{index .RepoDigests 0}}' ${REGISTRY}}/${{env.IMAGE_NAME}}/agent:${{ github.sha }} | cut -d: -f2)" >> $GITHUB_OUTPUT
         env:
           AGENT_IMAGE: ${{env.IMAGE_NAME}}/agent
 
@@ -65,6 +67,6 @@ jobs:
         uses: actions/attest-build-provenance@v2
         with:
           subject-name: ${{ env.REGISTRY }}/${{env.IMAGE_NAME}}/agent
-          subject-digest: ${{ steps.push.outputs.digest }}
+          subject-digest: ${{ steps.build.outputs.digest }}
           push-to-registry: true