fix: read the image id from a quiet build

[tas50]

Fixes #225 — nine years old and still reproducible.

The bug

docker build -q prints the id on a line of its own and nothing else:

$ docker build -q -f Dockerfile .
sha256:ab86ce908a36ffb7de411a72550e416a4d8c268570a0f7313c284c79344d6c0f

No step lines, no exporting to image, no writing image, no naming to, no Successfully built — none of the wording parse_image_id matches on. So every pattern missed, and build_options: -q failed after the image had already been built:

>>>>>> Message: Could not parse Docker build output for image ID

The fix

A line that is nothing but a digest is read as the id.

It has to be the whole line. Ordinary build output is full of #6 exporting manifest sha256:... done lines, so matching a digest anywhere would pick one of those instead — there is a spec pinning that.

The fixture is captured from a real docker build -q on 29.7.2, like the others in spec/support/docker_output.rb.

Confirmation

Docker 29.7.2 (Docker Desktop 4.87.0, macOS/arm64), Test Kitchen 4.1.1, real ubuntu-24.04 container, driver: {build_options: -q}.

Before:

>>>>>> Message: 1 actions failed.
       Could not parse Docker build output for image ID
       image_id=nil

After:

       Finished creating <quietbuild-ubuntu-2404> (0m1.40s).
       HELLO FROM PROVISIONER
       Finished converging <quietbuild-ubuntu-2404> (0m0.60s).
       image_id=sha256:9270df1e2707659a9fa55b8aac0ed440545736f175bcc90f2dad59af54234d0e

Ordinary (non--q) builds still parse — checked in the same run.

rake style clean; rspec 316 examples, 0 failures (3 new).