Add deploy key to SSH agent forwarding for Docker build in Google Cloud Build

cloudbuild.yaml

- name: 'gcr.io/cloud-builders/git'
  secretEnv: ['SSH_KEY']
  entrypoint: 'bash'
  args:
  - -c
  - |
    echo "$$SSH_KEY" >> /root/.ssh/id_rsa
    chmod 400 /root/.ssh/id_rsa
  volumes:
  - name: 'ssh'
    path: /root/.ssh

- name: "gcr.io/cloud-builders/docker"
  args:
    [
      "build",
      "--ssh",
      "default=/root/.ssh/id_rsa",
      ".",
    ]
  env:
    - "DOCKER_BUILDKIT=1"
  volumes:
  - name: 'ssh'
    path: /root/.ssh

availableSecrets:
  secretManager:
  - versionName: projects/$PROJECT_ID/secrets/machineuser-ssh-key/versions/latest
    env: 'SSH_KEY'