Steps specified by digest (w/o command) are broken

[imjasonh]

Expected Behavior

Steps can be specified by digest:

steps:
- image: ubuntu@sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d
  args: ['echo', 'hello']

(To experience the broken behavior, make sure you don't also specify a command)

Actual Behavior

Entrypoint lookup fails, because the entrypoint is resolved, and the resulting digest is appended to the input image name, which already has a digest. The result is a string like ubuntu@sha256:6e9...@sha256:6e9..., which is invalid.

Additional Info

The bug is here, which isn't covered by unit tests 🤦‍♂️

cc @mattmoor
/assign

[imjasonh]

The error looks like this when a step is specified by digest, without command:

conditions:
  - lastTransitionTime: "2019-12-03T20:29:20Z"
    message: 'Missing or invalid Task default/dep-update: translating Build to Pod:
      Error constructing resulting digest: a digest must contain exactly one ''@''
      separator (e.g. registry/repository@digest) saw: gcr.io/mattmoor-knative/pr-builder-70f5460f789e054d2fe41e583216fb39@sha256:5d7712b713364de13ddab7170b839a6c595af554621349b895a2a4ccbc1ed201@sha256:5d7712b713364de13ddab7170b839a6c595af554621349b895a2a4ccbc1ed201'
    reason: CouldntGetTask
    status: "False"
    type: Succeeded

(note the duplicate @sha256:... in the image name)