title: unknown blob on docker push to gcp
body: <p>I have a CI/CD build system running on GCP cloud build.
  It sporadically fails upon dock push with error: &quot;unknown blob&quot; (see below).
  Rerunning the failed job ends with a success result.</p>
  <p>The CI/CD is a Google CloudBuild job, that runs a shell script.
  The shell script runs docker build, and then docker push, so the built image is available locally after the build.</p>
  <pre><code>docker build -f Dockerfile -t gcr.io/XXX/YYY/ZZZ/test:latest-stage1
  docker push gcr.io/XXX/YYY/ZZZ/test:latest-stage1
  </code></pre>
  <p>Any ideas?
  Maybe I can add some additional commands to print some debug info for next time it happens?</p>
  <pre><code>Pushing image gcr.io/XXX/YYY/ZZZ/test:latest-stage1
  The push refers to repository [gcr.io/XXX/YYY/ZZZ/test]
  4eed37762d61: Preparing
  a68d95ff11ea: Preparing
  dddcf18e7786: Preparing
  e8c437b49d31: Preparing
  dea41e2ed72f: Preparing
  b660dabbb08d: Preparing
  3434e68efd32: Preparing
  efc45381b778: Preparing
  0902cfca6c4f: Preparing
  66f4f21514da: Preparing
  9e27e8abe53b: Preparing
  7b5ae180667a: Preparing
  613ab28cf833: Preparing
  bed676ceab7a: Preparing
  6398d5cccd2c: Preparing
  0b0f2f2f5279: Preparing
  b660dabbb08d: Waiting
  3434e68efd32: Waiting
  efc45381b778: Waiting
  0902cfca6c4f: Waiting
  66f4f21514da: Waiting
  9e27e8abe53b: Waiting
  7b5ae180667a: Waiting
  613ab28cf833: Waiting
  bed676ceab7a: Waiting
  6398d5cccd2c: Waiting
  0b0f2f2f5279: Waiting
  dea41e2ed72f: Layer already exists
  b660dabbb08d: Layer already exists
  3434e68efd32: Layer already exists
  efc45381b778: Layer already exists
  0902cfca6c4f: Layer already exists
  66f4f21514da: Layer already exists
  9e27e8abe53b: Layer already exists
  a68d95ff11ea: Pushed
  613ab28cf833: Layer already exists
  7b5ae180667a: Layer already exists
  6398d5cccd2c: Layer already exists
  bed676ceab7a: Layer already exists
  0b0f2f2f5279: Layer already exists
  4eed37762d61: Pushed
  dddcf18e7786: Pushed
  unknown blob
  </code></pre>